Terraforming another planet is an engineering problem (of course, not only an engineering problem), and in the end it's something that in theory can be accomplished as long as we "work it out".
Changing the human nature however, is not trivial by any measure.
It's very hard for individuals to change, and even more so for groups and even more so for societies. There are even those that say change in a person is impossible.
Whereas an engineering problem can (probably) be solved regardless of whether humans change or not.
One could even argue that advancing our technology has helped us change as humans (for the better hopefully), but the other way around doesn't necessarily hold.
Don't get me wrong, I'd love for us to be able to realize the damage we are doing to this planet, whether or not we colonize other ones (ideally I'd like both). However improving our engineering skills seems much more likely than improving our nature does.