I inquired about the status of my manuscript after 1.5 months since it
was still showing "With Editor " status.
I've found that six months tends to be about the average length of time before a paper in pure mathematics receives a referee's report, though it might take a few months less if the paper is especially short or non-technical, and a few months longer if the paper is on the technical side or is hard to find a referee for. Even when dealing with journals that I know for a fact explicitly tell their referees to get them a report within a few months (since I also have refereed for them) I've found that it's pretty rare to get a report before six or so months have gone by.
You've said that you inquired about the status of your manuscript after going 1.5 months without a report. I'm sad to have to be the one to break the news to you that, at least in the areas of math I'm familiar with, there's an excellent chance that the referee hasn't printed out your paper to start reading it yet.
Why is the Editor behaving this way with me? I am waiting anxiously
for my report as it matters a lot to me.
Remember, the editor and referee aren't being paid by the journal. So if they have to choose between spending time with your paper and spending time prepping their classes, writing a lecture, meeting with their students, etc, your paper is going to get pushed aside.
Also, and I think this gets forgotten too often, the editor and referees are people. Perhaps the past couple of months have been very stressful for them. Perhaps they have something major happening in their personal lives. Perhaps they're even sick in the hospital. (I once had an editor pass away while handling a paper of mine.)
The point of the above isn't that you shouldn't expect editors and referees to take your paper seriously. It's that you should remember that they aren't your personal employees. Sending a polite inquiry about the status of your paper every few months is totally acceptable, though you should probably wait until at least 3 or 4 months have gone by first. Sending near daily emails is not acceptable and is, I think, extremely unlikely to wind up with a positive result for you.
Finally, you should keep in mind that it's definitely possible that the referee is having trouble finding a referee for the paper. (You should take the online status updates like "With an Editor", "Under review," etc with a grain of salt.) Finding a good referee for a paper can take quite a bit of time if the paper is technical or in a niche area.
Are they avoiding my mails intentionally by not replying?
If you've been sending an abnormal number of emails then perhaps they are intentionally not responding. But it's also possible that they simply don't have anything to say. Perhaps after they received your initial email they reminded the referee about the paper and asked when they might expect a report. If you email them again two weeks later then there probably isn't much that they can even do. After all, if they start sending frequent harassing emails to the referee then they'll likely be told either (1) that the referee won't be able to complete the report in the timeline that the editor wants and that perhaps they should get someone else, and / or (2) that the referee won't want to referee any future papers for the paper. Either way it's a loss for the editor.
What is their aim? Do they want me to withdraw my paper? Should I
withdraw my paper because the Editor is behaving so badly with me?
I know that it's extremely important to you that your paper be published in a reasonable amount of time, but as I said above, you should keep in mind that most pure math papers take 6-ish months to get refereed, and many mathematicians have had papers that took years to get refereed.
Mu suggestion is that you try to remember that the editors and referees involved with your paper are almost certainly doing so voluntarily as a service to the mathematical community, and while they are committed to doing a good job, have their own full time jobs to worry about.
You can withdraw your paper, but this means you have to start the process all over again. And of course there's also the possibility that you simply get assigned the same referee.