If you intentionally carry out an action with the knowledge that property or people are likely to be damaged as a result of your action, you are morally at fault. It doesn't matter if the object or person that is injured was not properly tended to, you still performed that action knowing that a likely result is a damage of that property or person.
If you see a handgun laying on someone's table at their house, if you point that gun at them and shoot them with it you're still at fault for shooting them even though it was their negligence to unload the firearm that caused the injury.
The same can be said about them. Its their fault for taking that risk and riding my ass, they should've known better. Yes, if someone is killed/badly hurt I will feel sorry for them, but in no way will I feel like it was my fault. If for some reason I actually do have to stop, like for example to keep from hitting a kid that runs out in the road, then is it still my fault if the person behind me rear ends me and their child not properly secured is killed? no, obviously it isn't my fault, it is theirs entirely. The same goes for when I decide to brake check them. If they don't want their kid hurt, then they shouldn't take the risk of riding the ass of someone they don't know, and in addition to that, it would be their fault for not securing the child properly before going for a drive. I'm not going to take responsibility for someone else's dumbassery and ignorance.