Question: I am planning on making the main character of my writings to be an anti-hero. Some scenes are easy to keep consistent with his more cynical personality but I am having trouble determining when he can lean completely on a logical action and when a logical decision is far too selfish or morbid than he was intended to be.Answer
Maybe your character has the same dilemma and you've just stumbled upon your thematic tension? He could act differently in different situations, because he doesn't always know the right thing to do or what he wants to do. Maybe his inner conflict revolves around his own feelings about the selfish choices he makes?
Another possibility: according to Dramatica theory, main characters often have a "critical flaw" that undermines their ability to cope with a problem. This is balanced by their "unique ability," which is the thing that makes them the only person able to tackle the Story Goal.
For instance, if a compassion for others is your MC's unique ability, his critical flaw could be a selfish streak that prevents him from acting on his compassion (for a time).
Of course, these are just possibilities. I don't know your story well enough to give a concrete answer. Only you can do that.