1)
Why did he mention his eyes?
1.
Radak: The lack of Shemirah was due to the eyes; they stray, and a sinner follows his eyes - the eyes and heart 1 are two merchants of sin. Sha'arei Teshuvah (1:15) - the eyes atone for their sin via tears. It does not say 'Lo Shamarti', rather, "Lo Shamru" - they caused the sin.
2.
Malbim: Even though I have the sin of Bas Sheva, in which I veered after my eyes, my eyes already shed streams of tears for not guarding Your Torah. I already repented, and I was pardoned.
1
The heart entices to heresy. David's sin [with Bas Sheva] was due to his eyes. (PF)
