Murder is ethical leagues beyond spying and theft, and any group that does 'murder for coin' cannot pretend to be anything other than abjectly morally Evil. And that may drive away a lot of prospective thief types who are not interested in assassination. So unless your intention is to make this an unabashed Evil group, like an Assassins Guild that does less messy sidejobs at times, you may want to stay clear of assassinations completely so the group can at least stay somewhat neutral and this appeal to more shadowy character types than just immoral assassins.