In outline form Trempy's chain of thought runs:

I'm bored and very tired of having my dignity bruised - I want to kill that guy!

Yes, surely I could get away with it. He's an idiot and won't take my warning seriously. It would alert Nell to my difficulty in keeping the agreement I made with her.

No, wait, get a grip on yourself man and stop daydreaming! If I kill anyone that will break the terms I agreed to and Nell will have me killed.

He's arguing with himself like a dieter debating an ice cream. He knows he shouldn't but he wants to. One gets the feeling it is an argument he has been through a few times already.

Yes, the abbot/Roger is the man Nell designated as his jailor.

By the terms of the agreement he made with Nell, Trempy is not supposed to be killing anyone, or otherwise doing anything which might draw on his skills. He is supposed to be sitting in Repton doing nothing. Anything else will be taken as an effort to escape and/or regain some of his lost power.