I think of lying as speaking falsely with the intent to deceive, i.e. to cause someone’s beliefs to be confused or ignorant about reality.
In the case of checking the “I have read the terms and conditions” box, I’m not concerned that anyone is deceived into thinking I have read all of the preceding words rather than just some of them.
In the case of a murderer at the door, the problem is that the person is too confused already. I would do my best to protect life, but lying wouldn’t be the tactic. Depending on the situation, I might call for help, run away, command the intruder to leave, physically constrain them, offer them a glass of water, etc. I realize that I might be more likely to survive if I just lied to them or put a bullet through their forehead, but I choose not to live that way.
Kant’s reason that you described doesn’t sound very similar to mine. I agree with your critique of the proposition that lying is bad primarily because it increases the chance that others will commit violence.
My view is that the behavior of others is out of my control; they will frequently say and do things I don’t like, regardless of what I do. I’m only accountable for my own thoughts and actions. Lying is bad for my personal experience first and foremost, as I prefer to live (and die) without confusing my mind by maintaining delusional world models. My first priority is my own mental health, which in turn supports efforts to help others.
Similarly with regard to killing, my thinking is that I’m mortal, and my efforts to protect my health will fail sooner or later. I can’t escape death, no matter what means I employ. But while the quantity of my lifespan is unknown to me and out of my control, the quality of my life is the result of my intentions. I will never entertain the goal of killing someone, because spending my limited time peacefully is much more enjoyable and conducive to emotional health. Having made it to my car, I’ll just drive away.
It’s an interesting question whether someone who participates in fights to the death has a shorter or longer life expectancy on average than one who abstains from violence. But the answer is irrelevant to my decision-making.