How to avoid arguments and stop them from becoming harmful?

The core of an argument is almost always an emotional response (usually fear or anger or jealousy or insecurity or loneliness, especially when you're tired or frustrated), and so arguing the facts won't help (because that's not actually what's wrong). An argument is a symptom of deeper problems.

Communication is about empathy: gaining an understanding of what's going on with the other person. This is something that must be practiced by both parties in order for a relationship to work. When someone is arguing, they're really saying "you're not hearing me, and not seeing what's hurting me". Their issues could be valid or could be invalid or could be completely selfish but that's hardly the point. What they're looking for is recognition that you see them and value them.

Couples therapists help coach couples to see this deeper manner of connecting and communicating with each other, and help you practice (it really is a skill that you must learn and practice).

It's also good to have a regular regimen of relationship maintenance: https://www.youtube.com/watch?v=LLXX8wzvT7c

-Asking for forgiveness/apologizing. Even if you are right, a heated argument is always wrong.

-Learn to be quiet even if what is said hurts. Most of the time what hurts is actually not meant literally.

-Never generalize unless you are really calm and have thought about the matter and have many examples. It is very easy to turn a discussion on today’s grocery shopping into “you are always putting yourself first”, which is useless.

-Be eager to forgive.

Patience (aka live to fight another day), transparent motives and a feeling for what is appropriate.

Patience: in leadership trainings you learn about the importance of timing. A concerned party is not always receptive to an otherwise valid argument, so seeking (or facilitating) the right circumstances for a conversation is important.

Transparent motives: my motives should be clear to all concerned parties. This not only requires honesty, but transparency (eg. putting my cards on the table and allowing others insight into my situation).

Feeling for what is appropriate: I observe people losing brownie points by obsessing over details (wording, procedure etc). An honest introspection ("would the problem be solved if I won the argument over wording") can go great length to focus on what is important.

Edit: a meta comment, I am saddened by the fact that your question has received (at time of writing) 4 responses but only one upvote.

Once an argument starts, no one wins. The best thing to do is to be the bigger person and walk away. Deal with the problem with a cooler mind later. Easy to say, hard to do.

I remind myself of what the goal is and that it's not to "win" or score points.