First and Second Chronicles are pretty damned boring. So it's not surprising that they killings are too. Most of them are just the usual holy wars.
Still, there are a couple that deserve mention, just for the number of fatalities. In one battle, for example, God killed a half-million Israelite soldiers, and in another, a million Ethiopians -- the largest single, God-assisted slaughter in the Bible.
There are a couple other intersting holy wars in Chronicles. In one, God forced the enemy soldiers to kill each other, and in another, he killed 120,000 valiant men for forsaking him.
But perhaps the most interesting killing in these boring books is the one where God smote Jehoram with a bowel disease, until his bowels few out. The Lor works in monstrous ways.
Here's a list of the Chronicles killings: