最悪 【さいあく】is the word you're after, not 最やく, although that's what it sounds like. やはり = as expected. It is used to make exclamations: I really AM terrible/the worst/etc

It's a informal sentence meaning "I am the worst. (versus anther person)" it is used to compare yourself to another person who is better than you. Example: if I made a big mistake then I would exclaim: " やはり俺は最悪だ (Yahari ore wa saiakuda)"="I really am the worst..."