If you don’t like hitting people, it’s surprisingly difficult to train yourself to be okay with hitting people.
4th kyu(ish) is when a student is most likely to be a danger to another student (at least in jujutsu), because they’re getting good enough to be able to do real damage but don’t have enough control yet.
Bruises are pretty common, especially if you’re doing blocking techniques (or, generally, hitting each other).
If someone does a technique wrong, it’s not too hard for them to do a decent amount of damage to either themselves or the person they’re doing the technique on. Incapacitation techniques in particular can go wrong, because they’re based on putting a person’s body in stress positions.
Depending on what kind of martial arts being studied, a lot of techniques require less effort than people think they should.
Different types of martial arts and different dojos will subscribe to different levels of traditionalism, including how much English is spoken in instructions, what kind of beginning/ending ritual there is, and what is worn for day-to-day practice.
Disregarding military/military-esque training, which I’m not qualified to talk about, someone isn’t going to go from totally unexperienced to not-a-liability-in-a-fight-to-the-death in a matter of months. Please stop having characters go from being new to holding their own in high-stakes fights against people who wants to kill them in a matter of weeks.