それが良{い}いのか悪{わり}ィのか正{ただ}しいことなのか、少し考{かんが}える。
I interpret this clause from 僕のヒーローアカデミア as "As to whether that is a good thing or a bad thing or something correct, I will think about it a little."
Why is 悪{わり}ィ used instead of 悪{わる}い (i.e. why the り syllable after わ rather than the る syllable)? Also, why is the い at the end written in 小さいカタカナ rather than ひらがな?
As you can see in @Earthliŋ's answer that I linked above, this kind of sound change occurs quite often in colloquial speech.
/ui/ → /ii/
あつい → あちい・あちぃ・あちー
わるい → わりい・わりぃ・わりー
だるい → だりい・だりぃ・だりー
This is informal, and usually sounds masculine.
why is the い at the end written in 小さいカタカナ rather than ひらがな?
Katakana often look slangy, so I think they wanted to make it look more slangy.
Edit: As commented by @Sjiveru, the Kanatana ィ is also working as a cue for the slang pronunciation; 悪ィ with no furigana would still probably be read as わりい.