Saved Lucas Simms in Megaton... randomly died outside anyhow.

[urbanriot]

I did a quick skimming of what was involved in various areas of Fallout 3 and read about the whole bomb ordeal. Basically if I reveal Burke's intention to blow up the city, then tell Lucas Simms, then head to the bar with Simms, I can shoot Burke before he shoots Lucas Simms...

Which I did, successfully!

... yet there was Lucas Simms body randomly dead out in the city by the entrance!? What the heck!?

Is it impossible for him to stay alive even if you kill him?

[cruinne]

You have to kill him before he gets a shot off. Even if Simms is outside, Burke will still manage to shoot him dead, otherwise.

[QsuSanOzk]

Just save/load until you successfuly kill Burke and Simms is alive.
But there is no point in saving him actualy. He doesnt have any quests later and only thing he does if you save him just thanks you.

[obamafish]

You can also press the backtick key (`), click his body and type "resurrect". That will bring him back to life.