There's lying to your players because the outcome doesn't suit your plans - and there's "fudging". Two totally different things in my opinion ;)

I don't kill chars by chance. In a fight or a situation where they willingly (and often against advice) went themselves I don't have any problems. Let's take the example of climbing a castle wall. If a player decides to climb it even though they are told "It's dark and the wall is wet and there are guards above that might see you and you don't really have the equipment needed. Perhaps try another way into the castle?" - well, any dice that says the char tumbles to their death is good for me. But if, after a long and heroic fight, the chars have to climb down said wall because it's the only way out - I'm way more lenient. An accidental fall might hurt an already injured char further, leading to other problems, but it won't kill them.