1)
Why did he say "b'Tzidkasecha Tatzileni"?
1.
Radak: Due to Your virtue, and not due to mine. David knew that this is a punishment for the sin of Uriyah ha'Chiti - "Hineni Mekim Alecha Ra'ah mi'Beisecha" (Shmuel II, 12:11).
2.
Malbim: Save me amidst Tzedakah that You do for the sake of Tzidkos Hashem.
2)
Why did he say "Hate Elai Aznecha"?
1.
Malbim: Save me due to my Tefilah.
