Sometimes I kill monsters with booby traps (chandelior chains for example). This is good, it gives me fun 'acheivements' and bonus exp. Sometimes my follower or myself are close to the occuring calamity and I'm worried we are going to get hurt from it. Will we?
There are traps in the game which can damage you (and your followers). They are typically not activated by things you actively do (such as releasing a chandelier or smashing support beams) but instead on where you pass.
In contrast there are environment elements - such as the abovementioned chandeliers - that are triggered by the player and only damage enemies.
As a general rule, if it doesn't hurt you, then it doesn't hurt your follower(s) or human teammates either.