Nitya sanyasi Krushna meant to give up the ideology when doing any work like 'I am doing this, without me this could not have been done, am doing this expecting result for me' becauae that causes attachment and karma binds you.And a Hindu can be a 'nitya sannyasi' (ever renounced - as Lord Krishna said) even when performing all duties of the phenomenal world. He can be a saint even when killing people in a war if he is doing it in cause of his duty without hate and anger).
Arjuna is in a war and as a kshatriya its his job to end the adharma but he backs a little but as he is gudakesa who has won over his 5 indriyas/senses and without jealous of any thing Sri Krushna chose him to deliver bg..anyway point is killing without hate or anger does not justify killing