Some people are attracted to the Dark Side. And others try to avoid it but get caught in it by trying to do mitzvot and good deeds. This is not news. We find in several spiritual disciplines the fact that evil disguises itself as good in order to trick people. And sometimes people want to do good and simply have  a strong urge to do evil even while knowing that it is evil. But they don't try to justify it. They excuse it saying it is not that bad.

I don't have a good answer for the problem of the Sitra Achra. But some of the practices i talk about on my blogs are meant to help myself avoid the Dark Side.

My main model of goodness is what I saw in my parents home. And my parents were Reform Jews. And our home was one of wholesomeness and decency. So to a large degree I try to emulate them.
But I think we were not careful enough in the commandments, so I do try to keep the commandments of the Torah more than what was done in our home. But I try to do this with balance. For I see people that accept some ritual commands of the Torah (which in itself is good), but that leads them to ignore more important aspects of the Law--like loving one's fellow man  or honoring ones parents.

In any case, my basic ideas about avoiding the Dark Side are to talk with God like one talks with a friend, and be especially careful about never saying a lie or anything not even slightly not true. When I found myself sinking, I decided to hold onto this trait with all my might--never to say anything not true under any circumstances, and I believed that the strength of the truth would help me get through everything and hold me up.

A good link to this subject by Kelly Ross attack on morality