There's nothing wrong with knowing and training martial arts if we have a clear mind about ethics. What we do with that knowledge is what matters in the end.
Martial arts training may be quite useful to master our control over the expression of feelings and emotions. When you fight an opponent during training it's likely you'll get hit hard more than a few times. Observe your emotions then. It's a good training ground. The combat is always a simulation of sorts, since you aren't trying to kill each other. Life sometimes has events that resemble a fight. Things happen very quickly and so do emotions arise, feelings develop and so on and so forth. A fight is a very immediate experience. If our mind is well tamed, we will see violent emotions arising urging to revenge. It's possible to overcome them even when fighting, it is said. If our mind is not well trained, as soon as we get hit hard, we will try to act based on nasty emotions (pride, revenge and so on) instead of acknowledging our worse technique or unexpected luck of the opponent. Life has many similar situations. And as in life, it's hard to improve if revenge is our main drive. We can discover a lot about ourselves during martial arts training, but we must be willing to do so. Win or lose, the fighter must always leave the ring/mat with a feeling of appreciation for his opponent who is willing to endure training with him. When it comes to competition, things may be a little different and it's a matter quite individual. What are you doing there, really? Only the practitioner can answer that. I've practiced several martial arts for more than 15 years, my base formation being mostly karate. When I started training, none of the above was on my mind. That was the wrong way to go about it. I ended up training in a very hard gym where basically we would kick each other's asses to oblivion because of the kind of competitions we used to do (very similar to daido juku rules, but with ground fight). We used to complement karate with jiu-jitsu, boxing, kick-boxing and even taekwondo training, not because karate lacks good techniques, but because when we talk about sports events, and competitions are just that even with loose rules, karate is hard to use since its aim is causing real injury, really fast, and that makes it less sporty, unless you fight under the most usual shobu sambon tournament rules, which render karate to a poor a joke in my opinion.
Anyway, back then all that mattered was to become really fast when putting someone to sleep. There's no way about it. This is plainly against what Buddhist ethics.
Years later I realized all those missed opportunities I talked above. Is it possible to practice martial arts while being an ethical person? Sure! Is that the rule in most dojos/gyms?... Hardly.