What's he saying?
"Then hate me when thou wilt; if ever, now; / Now, while the world is bent my deeds to cross,"
If you are going to hate me eventually, do it now, while I am already suffering misfortune,
"Join with the spite of fortune, make me bow, / And do not drop in for an after-loss:"
Go along with what life is already putting me through, and crush me under the weight of your rejection
"Ah! do not, when my heart hath 'scaped this sorrow, / Come in the rearward of a conquered woe;"
Don't wait until after my general bad fortune has passed to hurt me yourself in a sneak attack;
"Give not a windy night a rainy morrow, / To linger out a purposed overthrow."
Don't make my misery last longer than it has to by dragging it out.
"If thou wilt leave me, do not leave me last, / When other petty griefs have done their spite,"
If you're going to leave me, don't do it after these other bad things have passed,
"But in the onset come: so shall I taste / At first the very worst of fortune's might;"
But hurt me right at the beginning of my bought of misfortune, so it won't get any worse;
"And other strains of woe, which now seem woe, / Compared with loss of thee, will not seem so."
All my other troubles won't seem so bad after losing you.
Why is he saying it?
Sonnet 90 continues directly from the previous sonnet, which ended, "For thee, against myself I'll vow debate / For I must ne'er love him whom thou dost hate." In the final couplet of Sonnet 89, the word "hate" proves a shocking contrast to the speaker's usual use of the word "love," and it is echoed here in the line one of Sonnet 90: "Then hate me when thou wilt; if ever, now." In the conventions of the sonnet, "hate" means a beloved's disdain more than the modern-day meaning of the word.
This sonnet also continues the downward spiral of loss and wretchedness that began with the sense of a final separation in Sonnet 87: "Farewell, thou art too dear for my possessing." However, the reality of the separation is up in the air. It is unclear whether it has already occurred, or whether it is impending. The use of the word "if" in line 9: "If thou wilt leave me," suggests that it is not even certain whether the fair lord has decided finally on the separation; it might be the case that the poet is worrying over a suspicion.
The imagery of a war is used to convey the speaker's feelings concerning losing his love in addition to all the troubles that already plague him. He is already under siege by "the spite of fortune," and he pleads with the fair lord to figuratively attack him with the bad news of separation now, rather than waiting until the end of the war to leave him. Lines 5-6, "Ah! do not, when my heart hath 'scaped this sorrow, / Come in the rearward of a conquered woe," give the impression that the poet has overcome the "sorrow" that fortune has beset upon him, but now the fair lord would sneak attack his figurative army from behind, as if he were a reinforcement for fortune's army.
The meaning of line 4 is unclear, though scholars think it likely has to do with gaming. The context of the sonnet suggests it might refer to war, in alignment with the war imagery, especially in light of the words "rearward," "conquered," "overthrow." The idea seems to be that the fair lord's rejection would be like an unexpected loss in a game, or a change of fortune in a battle whose outcome seemed to be determined in the speaker's favor.
Lines 11-14 clarify the reason for the speaker's insistence that the fair lord leave him now, rather than later: he would rather bear the worst blow first, so that whatever bad fortune follows won't seem so bad. The phrase "strains of woe" refers to different variations of sorrow, in different aspects of the poet's life. "Strain" also means the sound of a piece of music being played, like a tune of sorrow; for example, as it is used in 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!" (I.i.4-7).