Bava Kamma 91: When an Object Becomes a Weapon

Is there an assessment by the court to establish damages? Certainly, if the injured party is killed, the item is assessed to be a potential tool to kill, but what about if it's just an injury? Recall the "pit" that has a measure at which it's considered potentially killing (10 tefachim), but where less than that, the owner is only responsible for damages, not death. Also, is it permitted for a person to injure himself? What about cutting down his own trees? Under what circumstances are these acts that cause harm to oneself or one's property permitted?