In Breaking Bad, Season 3 Episode 12 (Half Measures), Jesse is incensed with the rival drug dealers for using children (in particular Tomás) to do their killing and selling. Gus orders the dealers "No more children" and orders Jesse to make peace. Subsequently, Tomás is killed (presumably by the dealers), which triggers Jesse in to an attempt to kill the dealers.
As Jesse is approaching the two dealers with his gun, Walt drives in from nowhere and runs over the dealers. One is killed instantly, the other seriously injured. Walt jumps from his car, grabs a gun and shoots the second dealer dead.
Why does Walt do this? Is it simply because Jesse would likely have been killed during the showdown, or is it because he didn't want Jesse to become a murderer like he already was? If anything, I thought Walt may have been motivated to simply let Jesse die, due to Mike's "No half measures" speech.