While it is certainly feasible that an armed person could have stopped or slowed down the madman in Colorado, the situations were completely different. Holmes appears to have had a break with reality, and in addition was armored. Not 100% armored, but quite a bit. His arms, thighs, and face were available, the rest not so much. Add to that the darkness, tear gas general panic and pandemonium, and you have a situation where a person with a small handgun would have little effect.
The guys the older gentleman shot in the internet cafe were more ordinary thug punks who were completely in touch with reality, but were also stupid and cowardly, so that when the first BANG happened they proceeded to skitter on out of there and floor swim over each other.
The internet cafe idiot punks were there for an easy take, and had no intention whatsoever (apparently) of standing their ground and receiving fire if necessary to accomplish the goal. The Colorado madman had the intention of inflicting mass death and pain, and was suited up to give and receive fire. Once he did what he wanted he went outside and later just gave up. That part was a bit odd.