Why does it say "Techate'eni"?
Radak: This is like "Herev Kabeseni" (verse 4). Avon to the Nefesh is like a stain on a garment, or Tzara'as on a body.
Malbim: This is another reason why it is proper to remove my sin - I already suffered afflictions and got atonement via Nega'im, and I was purified via Ezov, like the law of a Metzora 1 . This should be the Taharah from Chet, and cleanse from Avon (verse 4).
Yuma 22b: David has Tzara'as for six months.
Why does it mention "Ezov"?
Rashi: This is like Taharah of a Metzora and one who is Tamei Mes.
Radak: The final Taharah is via sprinkling with Ezov, so purify me from (remove) my sin.
Malbim: Ezov (hyssop) symbolizes physicality being submissive, so it will not continue to lust and be haughty. The Nega itself illustrates the Avon, i.e. crooked thinking, which is the inner Nega; the stain stuck to the Nefesh breaks out via the skin - "veha'Tzaru'a Asher Bo ha'Nega" (Vayikra 13:45).
What is the grammatical form of "v'Albin"?
Radak: It is Po'el Omed (the action applies only to the subject), even though it is Hif'il. The same applies to "Hilbinu Sarigeha" (Yo'el 1:7).
Why does it say "mi'Sheleg Albin"?
Malbim: The stain, a Baheres, is white like snow. If he turns totally white he is Tahor, for then it is not like a stain; it is the appearance of his skin. It is as if David turned totally white like snow - this is cleansing from the stain.
