That was really about him being an Avenger, and following the rules, because without rules the guy was always kind of lost. Plus he's just sort of nice, down at the core.
But now the team doesn't have rules...and he's had a bad few years (dead wife, jail, remembers getting jacked into assorted alternate realities, died himself a few times, etc.). And then, well, the Skrulls. I think it's a reasonable progression; now that he's crossed that line and found out that he's comfortable with it when necessary, why not take out the biggest SOBs on the planet?