Evidence: Russia completed the theft of Crimea(while denying their active invasion of it) and immediately built up forces for another attack while denying it would ever attack Ukraine again all the while talking about how Ukraine isn't a real country. There is no evidence Russia will stop until it controls all of Ukraine as they have said they wanted to do many times.
West and Trump want to trust Putin...a verified(many times over) liar and willing to break agreements and treaties at any moment.