I have to say, if I'd been treated the way you had, and was worried that perhaps my radar wasn't quite strong enough to pick up on a bad one on its own, I might well have done what you have. Particularly if there were children to think about. So don't be too hard on yourself
Secondly, yes you should dump, and all you have to say is that you have changed your mind. You aren't in the right place to date, it's too much too soon, you think you need to be alone for a while. You don't need to give him a 'valid' reason- all that is perfectly fine. It is DATING. You can change your mind just because you want to, because it's just about whether personal attraction is there or not. You're not trying to get out of a contract or anything.
To come back to the snooping, though. What's happened here? Well, you've met a guy, it's been great, its moved really fast, and you needed to know whether you could trust him, and you found that you probably can't. Why did you need to snoop? Not because you can't trust- but because of the bit in brackets. That's the problem, not your trust issues.
Talking of marriage after 5 months? Insanity. Red flag.
Too fast. After 5 months, you do not know someone. it's not possible. Ergo, only a fool trusts a person they've known 5 months. Only an absolute fool listens at all to someone who talks of marriage after 5 months. You're not a fool. But instead of reacting by slowing things down, you panicked. Instead of taking control in that way- overtly, you seem to have defaulted to letting him set the pace but secretly trying to protect yourself.
Do you see?
A person with better boundaries would have taken a step back when it started getting intense, telling themselves that he was beginning to seem a bit suss. And lo- he IS suss. Bored pseudo-romantic flirty scalp collector. Exactly the same as he's been with you, but you couldn't see it.
Next time, don't reach for the keylogger... just remember that it takes a couple of YEARS, not months, to know somebody. Within that time, keep your powder dry, get to know them properly before marriage is spoken of, and I reckon you'll learn to weed out the rubbish using your own instincts.