I'm shocked that your manager is willing to wait a couple of months to separate you two, and not do anything more in the meantime. Did your manager not give you any more information on that part? Also, "he is a greater asset to the company", did someone tell you that or are you assuming this?

@Kozaky I think that, he built all the systems here, some coworkers who know about all this tell me that I don't need to worry about that and they wont fire me but I don't know because its a small company and if it comes to me or him, it will be me who get fired because he have no replacement (I think I can replace him but I don't think they know that and will do that anyway)

@SouravGhosh I hope not but its a small company, and they also (from what I gathered from coworkers) got attacked (verbally) by him, and want to fire him but he worked here for 10+ years..

@teego1967 the problem with him is that he is 50 years old and tired (he told me all that) and want them to fire him.. so I don't think he will change his behavior. maybe I am wrong but he told me those stuff

Sounds like they may fire him anyway. HR and management sometimes cares when people start getting crazy

@Anon222 You handled the situation much better than I would, I would have gotten into a yelling match with him and he would have never seen the end of it. Well done for being the bigger person.

Explain that his "code" - it wouldn't pass for that in my office - should have version control so he should just be reject merge requests. Tell him his code is hacky garbage full of tehcnical debt. Tell him you know he's incompetent and can't do his job.

@Anon222 If this guy just wants to get himself fired, then verbally abusing co-workers seems a rather needlessly pointless way of doing this when he can just not bother turning up to work (or just resign instead). Have you asked him why he's using this "abuse" method over more conventional methods of leaving a company?

@Steve you really think that attitude will help the OP get out of trouble? To me someone escalating THAT bad is someone I don't want in my team, regardless of who's right....

@Patrice OP isn't in as much trouble as this senior dev. Just need to check him before you wreck him. Behavior like this from a senior exudes stupidity and incompetence.

@Steve Well the OP thinks they are in trouble with the senior dev. I don't see that changing by challenging said senior dev. I agree the dev is likely not as good as he thinks he is, and likely not worth the trouble from the company. It's still no reason to escalate this. Being professional is often being able to bite down on what you feel like you should say. This is one of these instances. Even if OP is right, escalating that way, to me, shows a sign of someone who will have issues working in a team, as soon as the stakes are high and people get stressed.

@Patrice The senior dev is the issue. Dude needs to be replaced. Not saying what you should say isn't professional. It's cowardly. In order to be professional you need to say what should be said in a way that people will listen to. If people decide that they're done listening, you need to make them listen. If it doesn't need to be said then don't say it. This "Senior" dev is destroying the code base with poor practices. Eventually this will cripple the company.

@Steve I agree, but you think "be professional" is "Tell him his code is hacky garbage full of tehcnical debt."? Cause that's what you said to say, which implies you think that's the professional way to say it. that part I can't stand behind. I agree with all you said about the potential damage of such a dev. God knows I had to help codebase AFTER such devs passed through it.

Your question could us a lot more precise language. For instance, when you say "So what I did is add 1 line of code (assignment) that he forgot", what does that mean? Did you edit the file he was working on? Did you create a fork and add the line to the fork? Code isn't like a painting. If I go into the Louvre and throw paint on it, it's ruined. If I edit a program, then assuming proper version control, the original isn't affected at all.

He sounds insecure to me.

I wonder if that behaviour has also a gender discrimination component in it. Are you both classified as the same gender?

"And he is a greater asset than me for the company" - no, just no. Employees that create a toxic work environment are a liability, not an asset.

Is this a small company without proper version control and a code repository? That's going to become a problem one day, probably a big problem.