IPA: /ˈiːvɪlər/
KK: /ˈivələr/
More wicked or morally wrong than something else.
His actions were eviller than I had ever imagined.
'eviller' is derived from the comparative form of 'evil', which comes from Old French 'evil' (meaning bad or wicked) and the suffix '-er' (indicating a comparative degree). Thus, 'eviller' means more evil or worse than something else.
Think of 'evil' as the base meaning, and remember that 'eviller' indicates something that is 'more evil' or 'worse'.