I think maybe people's idea of what you need to do to be evil is out of step with what it means to be evil. Being evil isn't a record of your deeds, it's your worldview, if you've never hurt a fly before in your life, but you think Might Makes Right, then you're evil. Just because Astarion or Shadowheart haven't twirled their moustaches while murdering children doesn't mean they haven't espoused evil ideology.
This is not how things work. Morality is for social action, and you're judged for your actions using your culture's dominant morality, not for your motivation. In some cultures it doesn't even matter why you do something at all for how you're judged, but all cultures judge certain actions like killing one of your own group. If "good" and "evil" is about morality, then action counts and ideology alone does not, and to say that someone "is" evil or good does not make sense as a rule.
Which means that as long as you're absolutely determined to use that phrasing that someone "is" evil, they're only evil insofar as they do evil. Doing evil is what makes you evil, if anything does. How you think might cause you do to evil, but it won't make you so on its own.
In any case, ideology doesn't matter much for how I deal with companions. It's cause for suspicion, all right, but not for judgment.
I think modern legal philosophy is the exact opposite of what you say. Conviction for almost every serious crime (other than drug possession, in the U.S. anyway) requires proving an action that causes social harm and an intention to do the act. You cannot criminally punish someone for an act alone and you cannot criminally punish someone for a thought alone because history has shown that trying to punish thought crimes leads to bad outcomes for society. However, it is the intention that makes the act a crime, with the act merely being the expression of your "evil."
