Why is setting someone on fire if they attempt to murder you with machetes considered reckless use of lethal force?

A very strange question but as a former police officer, I can answer it. The laws of self defence do allow you to use reasonable force to defend yourself, its true. There is also a thing called "instant arming" so thats when you pick up whatever is around you and use it as a weopon to defend yourself. That's probably what you would be using if someone broke in. So I am in my kitchen now, someone breaks in with a machete and I grab a knife from my kitchen block, thats perfectly legal. I am not expected to use minimal force, thats not practical, so if I kill them while defending myself like that, I should be fine and there have been cases where people have fought back in their homes, killed the intruder and not been convicted. Now we come to setting people on fire. I really cant see many circumstances where you are going to be instant arming yourself with something thats going to do that. Remember, you have to argue you were defending yourself in the moment, not due to a threat or something like that. So while lethal force as self defence can be legal in the right circumstances, I cant think of any where setting someone on fire would fit the bill.