They also that render evil for good - They whose characteristic it is to return evil for good, are opposed to me. This implies that those who were now seeking his ruin had been formerly benefitted by him. They were persons who cherished no grateful recollection of favors bestowed on them, but who found a pleasure in persecuting and wronging their benefactor. Compare Psalm 35:12-16. "Are my adversaries." Are now opposed to me; have become my enemies.
Because I follow the thing that good is - This properly means, Because I follow the good. The Hebrew word rendered "because" - תחת tachath - means properly the lower part; what is underneath; then, below; beneath. The idea here is, that the "underlying reason" of what they did was that he followed good, or that he was a righteous man; or, as we say, This was "at the bottom" of all their dealings with him. Sinner as he felt he was (and as he acknowledged he was) before God, and true as it was that his "sickness" was brought upon him by God for his sinfulness, yet the reason why "men" treated him as they did, was that he was a friend of God - a religious man; and their conduct, therefore, was sheer persecution. We may, with entire consistency, be very humble before God, and acknowledge that we deserve all that He brings upon us; and yet, at the same time, we may be sensible that we have not wronged men, and that their conduct toward us is wholly undeserved, is most ungrateful, is sheer malignity against us.

Because I follow the thing that good is - The translation is as bad as the sentence is awkward. תחת רדפי טוב tachath rodpi tob, because I follow goodness. There is a remarkable addition to this verse in the Arabic: "They have rejected me, the beloved one, as an abominable dead carcass; they have pierced my body with nails." I suppose the Arabic translator meant to refer this to Christ.
None of the other Versions have any thing like this addition; only the Ethiopic adds, "They rejected their brethren as an unclean carcass." St. Ambrose says this reading was found in some Greek and Latin copies in his time; and Theodoret has nearly the same reading with the Arabic: Και απερῥιψαν με τον αγαπητον, ῳς νεκρον εβδελυγμενον· "And they cast me, the beloved, out, as an abominable dead carcass." Whence this reading came I cannot conjecture.

They also that render evil for good are mine adversaries; because I follow [the thing that] (o) good [is].
(o) He would rather have the hatred of all the world, than fail in any part of his duty to God.

They also that render evil for good are mine adversaries,.... See Gill on Psalm 35:12;
because I follow the thing that good is; or "follow good"; a good God, whom his soul followed hard after, Psalm 63:8; the good Shepherd of the sheep, who led him into green pastures, whither he followed him, Psalm 23:1; the good Spirit of God his guide, whom he walked after, Romans 8:1; good and holy men of God, whom he took for examples and copied after; and every good work, which he pursued with eagerness and pleasure; and all this drew upon him the hatred of his adversaries.