Would it be wrong to snoop on my SO's old texts?
Yeah... Why would you want to see them?
There's someone she used to talk to, and I want to find out what she said to them.
You shouldn't look at or expect to see their old texts. That's private things and an invasion of privacy.
I want to make sure she's not being hurt by this guy.
That's a great thing to want, but invading her privacy is going about it all wrong. She is more likely to open up to you if you aren't invading her privacy.