The psalmist has admitted his own poverty in verse 1 and in verse 2 is appealing to God on the basis of his trust in God. In verse 3 he is appealing to God on the basis of divine mercy. In verse 5 he appeals to God's readiness to forgive and grant free grace to any who call on him. Therefore, the psalmist is not claiming he is holy in the sense of merit. The context doesn't support it. I think John Calvin had the right idea. The word "holy" here means "kind" or "meek". Calvin's point is that the psalmist is making an argument for God to preserve his life because he hasn't done anyone harm - ie, the psalmist's life is in danger because he is being mistreated.