Is it skillful to not intervene in his acts of killing, knowing he will not understand?
If you wanted to, how about just saying, "Blessed are the merciful", see how he understands that?
I wonder if perhaps he'd understand better if you spoke from within the context of his (Christian) scripture.
I propose you shouldn't want him to perceive "conceit" however -- i.e. his perceiving you as comparing yourself to him, as your saying that you are superior.
See How are 'conceit' and 'identity-view' not the same? for some description of conceit.
Instead it might work better if he perceives "love" (or mercy, etc.) which he is inspired to emulate.
What is an appropriate response to him asking if I "love Jesus only" or if I'm a "good Chrisitan"?
I don't know, how about -- "Jesus said to love God, and love thy neighbour".
I suggest that, not because it's a Buddhist dictum, but because I reckon you might want to speak using someone else's language in order to communicate with them.
Be careful not to lie, though.
Incidentally there are said to be four ways to answer a question
First the categorical answer,
then the qualified,
third, the type to be counter-questioned,
& fourth, the one to be set aside.
- "Yes" or "no"
- "If when you say 'Good Christian' you mean 'X' then etc."
- "How do you think you would recognise a good Christian?"
Anyway you might think you're required to give a categorical answer, but there are other ways to answer which might be better.
Should I not worry about un-deluding his mind to help him understand the Dhamma?
You asked about "skilful". One of the words translated "skilful" is kusala, try this essay for example, Kusala and Akusala (which seems to have been written for Christians, but anyway), one of the things it mentions is,
In the early Buddhist discourses, individuals are classified into four groups, in the following manner :
- The individual who pursues neither his own (moral) well-being nor others’ (moral) well-being
- The individual who pursues others’ (moral) well-being but not his own (moral) well-being
- The individual who pursues his own (moral) well-being but not others’ (moral) well-being
- The individual who pursues his own (moral) well-being as well as others’ (moral) well-being
I think that un-deluding another's mind is what a bodhisattva might aspire to.