To shoot oneself in the foot means to make a situation worse for oneself without meaning to, without intending to.

搬起石头砸自己的脚，意思是无意地让情况变得更糟。

He's assuming that there's a good deal of splattering when one shoots oneself in the head, and he's loath to leave more of a mess behind than he has to.

欧维猜，往自己脑门上来一枪会溅得挺厉害，没必要弄得太乱。