Lamon is late to the vampire-killing party, and what a shame it is. He jumps on the bandwagon long after Abe starts a law office with him in 1852 (9.14). See, Abe may work with the guy, but he only tells Lamon about vampires when he needs a big dude to go with him to assassinate Jefferson Davis almost eight years later (10.84).
We're thinking Abe probably should have thought of having Lamon tag along sooner. After all, "He was big, tough, handy with a gun, and fiercely loyal—and his help was desperately needed" (10.167).
And yet, the one time Abe goes out with Lamon (and Speed) to try to kill Jefferson Davis, they don't do so well. By which we mean… they fail. So maybe Lamon is not as handy with an ax as one might expect, given the above description.
In fact, Lamon is much more effective as anti-vampire bodyguard. Since he's the only human bodyguard who knows about the vampires, Lamon is the most nervous, vigilant guy on Abe's staff. (At least, when the vampires who guard Abe get excited, their hearts don't race—because they're vampires.)
But Lamon's private security talents are both his gift and his downfall. He's a very good, very loyal bodyguard: "White House staff would grow used to seeing Lamon patrol the White House grounds after dark, or sleeping in front of the door to the president's bedroom" (10.167). (That's good, because no one gets in without waking Lamon. But it's not so good because Abe can't sneak down for a midnight snack.)
But because he's so dedicated and Abe doesn't want any other bodyguards at Gettysburg, Lamon burns out. He tells Abe that he can't bodyguard him anymore—which means that no one is there to stop Booth when he comes to kill Abe (12.80). If only Armstrong was still alive. Or if only Abe were friends with Henry still. Or if only Batman had come through time to save Abe.