She wouldn't have gotten away with it
(First of all, let me preface this post by saying that yes, I do understand this is a fiction movie. I actually quite enjoyed it. But I felt like pointing out how it's nearly impossible that she would get away with killing her husband.)
1. Mitch is talking on the phone with his current girlfriend about going to San Francisco because his daughter is there and his wife has gone missing. I'm sure the girlfriend would add in her two cents about how suspicious it is that Slim showed up at the house and then he ended up dead.
2. When the police go through Mitch's house afterwards, I think they would find it odd that knives were in the refrigerator drawers and that guns were in odd places. That would be a huge red flag.
3. Just look at Slim's clothing. It's loose-fitting and athletic, like she was preparing for a fight. One could argue that she wore those clothes in case he attacked her, but she also had lotion (or whatever that stuff is) on her skin. This also suggests that she was preparing for a fight. Perhaps the police wouldn't notice at first, but after noticing other things, I think they would take into account her appearance.
4. She cut the phone line. Why would she do that unless she wanted all calls to be prevented, including in the event she herself needed to call 911? I'm pretty sure the police would notice the snipped line.
5. The autopsy would show that punches were made by hands with sharp rings or similar objects. That suggests that someone WANTED to hurt him and badly.
6. Someone would find the bag she threw in the river. It has to happen some time. That's a very interesting set of tools, so if she wasn't caught in the beginning, the police would probably wonder why someone threw this bag in the river, why and how they were used, and what crimes/incidences could have been made using those tools.
7. The fingerprints on the letter are probably not substantial enough to suggest he really took a good look at them. There would be none of his fingerprints on the back of the letters, so the police would know he didn't handle the paper from an envelope. They would only be on the front, probably not very many prints at all, and probably smudged because he fiddled around with the paper instead of firmly holding it.
Besides, is there any additional evidence of any letters? He had to have written to her, wouldn't he? Those are definitely letters that you keep. The police would notice a lack of phone calls, too.
8. If the cell phone call that Mitch made was put through at all, it would record that he called 911 and that it was cut off. Couple that with the tools in the bag, I think the police would get the idea.
9. If Slim suffered "intense physical abuse," as she wrote in her letter, why would she got to Mitch's house, unaccompanied? If she were going to meet Mitch face to face, wouldn't she more likely go to a crowded place and probably bring someone for protection? Even if she did have evidence of self-defense training, it's a little odd to go to a dangerous man's house by yourself when you know he's likely to try to beat the crap out of you.
(I don't take into account the amount of injuries she had because she probably had quite a few. He hit her a couple times, threw her against the wall, choked her, hit her with a lamp, etc. I don't think his and her injuries would necessarily show murder, just all the other stuff I just listed..)