Sonnet XC

Then hate me when thou wilt; if ever, now;
Now, while the world is bent my deeds to cross,
Join with the spite of fortune, make me bow,
And do not drop in for an after-loss:
Ah! do not, when my heart hath 'scaped this sorrow,
Come in the rearward of a conquered woe;
Give not a windy night a rainy morrow,
To linger out a purposed overthrow.
If thou wilt leave me, do not leave me last,
When other petty griefs have done their spite,
But in the onset come: so shall I taste
At first the very worst of fortune's might;
   And other strains of woe, which now seem woe,
   Compared with loss of thee, will not seem so.

 This sonnet continues directly from the previous one. It is a threnody of forthcoming woes which threaten to obliterate the poet entirely. Some doubt still remains as to the reality of the separation and loss. Has it taken place, is it a certainty over the next few days, has the loved one declared his intentions, or is it just a rumoured possibility that has been discussed in jest, or in earnest, in the privileged circle of the few initiates?

It echoes also sonnet 87, which has only recently oppressed us with the finality of separation - Farewell, thou art too dear for my possessing. So that we feel ourselves to be speeding down an ever decreasing spiral of loss and wretchedness, as the beloved youth frees himself from the shackles of love and no longer wishes to continue the association.

The 1609 Quarto Version

THen hate me when thou wilt, if euer,now,
Now while the world is bent my deeds to croſſe,
Ioyne with the ſpight of fortune,make me bow,
And doe not drop in for an after loſſe:
Ah doe not,when my heart hath ſcapte this ſorrow,
Come in the rereward of a conquerd woe,
Giue not a windy night a rainie morrow,
To linger out a purpoſd ouer-throw.
If thou wilt leaue me, do not leaue me laſt,
When other pettie griefes haue done their ſpight,
But in the onſet come,ſo ſtall I taſte
At firſt the very worſt of fortunes might.
   And other ſtraines of woe, which now ſeeme woe,
   Compar'd with loſſe of thee,will not ſeeme ſo.

Commentary

1. Then hate me when thou wilt; if ever, now;
Then hate me when thou wilt - the phrase seems to take up the final line of the previous sonnet:
For I must ne'er love him whom thou dost hate.
Then is more or less equivalent to 'If that is the case, as I have just described it, then hate me etc.'
when thou wilt = when it pleases you. But the thought is immediately corrected, for the pain of separation, if it is to occur, is best endured immediately. The uncertainty of his position, whether or not he still is loved, and whether or not he should cut his losses immediately, thereby preventing a long drawn out agony, carries over from the previous two sonnets.
2. Now, while the world is bent my deeds to cross,
bent = intent on, determined to. But also with the suggestion of 'crooked, perverted'.
my deeds to cross = to thwart my plans and activities. It is not known if these references to setbacks and fortune's spite are attributable to any particular event in Shakespeare's life.
3. Join with the spite of fortune, make me bow,
Join with the spite of fortune = Ally yourself with the rancorous ill will of Fortune. Spite is linked to Fortune in Sonn 37:
So I, made lame by fortune's dearest spite, 37
and OED gives an early example of the phrase from 1562: With a lustie manly courage he defied the spight of Fortune. The meaning is A strong feeling of .. hatred or ill-will; intense grudge or desire to injure; rancorous or envious malice. (OED.2.a.). Spite of line 10 has the more general meaning of 'malicious harm and injury'.

make me bow - i.e. crush me under the weight of your disfavour; make me humbly bow and bend the knee in submission.

4. And do not drop in for an after-loss:
drop in for an after loss - the phrase is not properly understood. It may be a gaming metaphor, or, in view of rearward, conquered, overthrow, onset, loss, which follow, a military one. SB gives an extensive note with details of slightly later examples from gaming and warfare. The general idea seems to be that of an unexpected loss resulting from a sudden hazard or cast in a game, or an unforeseen development after a battle, when the result was already supposedly determined. The modern meaning of drop in, 'to call on unexpectedly, or casually' is not relevant here, and is not attested until much later.
5. Ah! do not, when my heart hath 'scaped this sorrow,
'scaped = escaped.
this sorrow - presumably the sorrow of being thwarted at every turn by fortune, as in line 3.
6. Come in the rearward of a conquered woe;
The imagery is that of a surprise attack in the rear of an army. However the strict sense seems to imply that the conquering has already been done, and by the poet, who has managed to master his sorrow, rather than the sorrow mastering the man. This new attack reverses the situation and lays low the poet, who is already struggling to survive. Or perhaps it means that the army of woe(s) which seems to be conquered, is suddenly aided by reinforcements from the rear, which threaten to overcome the speaker entirely.
7. Give not a windy night a rainy morrow,
'Do not heap sorrow upon sorrow' seems to be the obvious meaning, although metaphors connected with meteorology can be ambiguous. Both cases here are of bad or ruinous weather.
8. To linger out a purposed overthrow.
To linger out = to drag out (the agony), to cause to continue (something unpleasant) for an excessive time, to delay. Cf. Lear:
................He hates him
That would upon the rack of this tough world
Stretch him out longer. KL.V.3.513-5.

a purposed overthrow = a defeat which is planned (by your rejection of me).

9. If thou wilt leave me, do not leave me last,
If thou wilt leave me = if you are determined to leave me. The uncertainty still remains.
10. When other petty griefs have done their spite,
other petty griefs = other sorrows and disasters, which, in comparison with the possibility of you abandoning me, are petty.
have done their spite = have injured me as best they can. See the note to line 3 above.
11. But in the onset come: so shall I taste
in the onset = in the first wave of the attack. A continuation of the military metaphor of lines 4, 6 and 8.
taste = experience.
12. At first the very worst of fortune's might;
fortune's might - Fortune was considered to be all-powerful, but endlessly fickle.
13. And other strains of woe, which now seem woe,
strains of woe = species, types, varieties of sorrow. I.e. the sorrows predicated as happening because of Fortune's reversals. With a hint also of the distress (strain) caused by the loved one's imminent departure. strains also could refer to 'strains of music', hence 'other tunes of sorrow'. Compare for example from Twelfth Night:
That strain again! it had a dying fall:
O, it came o'er my ear like the sweet sound,
That breathes upon a bank of violets,
Stealing and giving odour! TN.I.1.4-7.
14. Compared with loss of thee, will not seem so.

All other sorrows, though large at the time, are considered to be minuscule in comparison with loss of the loved one.