What should Hashem see in my eye?
Rashi, Targum Yonasan: The tear of my eye.
Radak: This is like b'Inyani (my case or matter). Also Chazal said 'me'Ein Kol Brachah u'Vrachah' (Brachos 29a), i.e. me'Inyan.
Why is "b'Eini" written with a Vov?
Radak: This hints to b'Oni, i.e. b'Anyi (my affliction).
Why should good come from the curse?
Malbim: One should not expect good to come from what Hashem decreed to be punishment for a sin. It suffices that the sin is pardoned! If evil above what was decreed comes on him, this can be afflictions of love; good and Tzedakah can come from them. The Navi said "Hineni Mekim Alecha Ra'ah mi'Beisecha" (12:11), i.e. Avshalom's pursuit. This was decreed, good will not come from it. The Navi did not mention this curse (it was not decreed) - perhaps good will come from it.
Why is "Kilelaso" written with a Yud (in some texts)?
Radak: It can be read Kilelasi or Kilelaso; the matter (curse) can be attributed to the subject or the object. The same applies to Tefilah - "b'Veis Tefilasi" (Yeshayah 56:7), "Shamati Es Tefilasecha" (Melachim I, 9:3).
Why did he say "ha'Yom ha'Zeh"?
Malbim: Today my sin was atoned for through evil pursuing me from my house. Shim'i's curse is added - perhaps it will be considered afflictions of love! It says about 'those who are insulted, and do not insult in return; they serve from love [and are happy with afflictions]' (Shoftim 5:31) "v'Ohavav k'Tzeis ha'Shemesh bi'Gevuraso" (Gitin 36b).