What?
A guilty conscience.
I didn't see Shane quite that way. Not being familiar with gunfighters (LOL!) I assume that Shane would be seen as a challenge. Sure, many would be afraid of his skill, but there would be others who would want to prove themselves.
Shane knew he could be deadly. And if pushed he would have to be. So he tried to avoid it, kind of in the same way a full grown man would avoid a fist fight with a young hotheaded kid. That kind of skill brings a responsibility and I saw Shane as always weighing his honor and the honor of those he loved against the responsibility he felt to use his skills in a moral way.