I mean, that guy was with Tommy from (almost the beginning). The fact that he would slowly catch an attitude and betray Tommy was difficult to think about.
So what if Lance was with Tommy from the beginning?
He was working with him simply because to help each other in killing Diaz.
Though, Tommy always treated him as someone he had to make him realise for all the mistakes than trust closely enough. Tommy had to focused to establish himself and the gang. While, Lance frequently felt insulted because of the Tommy's attitude on a number of occasions, thus he never really trusted Tommy, more than looking after his position and respect in the gang for example:
> In mission Bar Brawl - "I feel you're not giving me a fair slice. And more than that, you been embarrassing me in front of the boys. I can't have that"
> Further, in mission Cop land - "Keep your mouth shut there Tommy, 'cause I ain't got time to talk. I ain't interested in what you got to say. Why should I be? You don't care about me, do you? You gotta look after me a bit better. Give me a fair slice, you know... "
Basically, after playing the missions you get the idea that the scenario was building for the betrayal to come, but only on the right moment, and opportunity.
So, in my opinion, Lance can't be considered as the biggest backstabber, it was quite easy for him to break the partnership.
It wasn't shocking to me.