I have to imagine the skills outside of the basics of "killing things and ensuring they stay dead" for mercenaries and assassins are transferable. I'm sure assassinating someone takes a bit of planning, which means you may possess a good mind for planning, goal setting, and ensuring a task is done quickly and efficiently. I also would think you'd have to possess some level of physical skill to be a hired gun so you may be able to lift a decent amount of weight, which seems like a random requirement I've noticed, the ability to lift a minimum weight requirement (I've seen 25 pounds a lot. This seems especially true for any position relating to stocking inventory). Chances are you know how to follow directions but also how to adapt to situations as needed since it seems unlikely most targets would merely sit there and wait for you to do your job. You'd likely also have good observational skills, a good sense of your surroundings at all times, and the ability to pick out behavioral patterns. A modicum of patience seems like it would be, if nothing else, a learned skill that could come from your previous job.
Honestly, if someone hadn't already mentioned engineering, and if I knew your personality more, I'd almost suggest working with the children in some capacity, perhaps with that youth center I've heard about. Ultimately, I think it's about how you spin the skills you learned from your previous job and how you can apply them to what you may want to do know (which seems to be engineering, and that is a good field).