I am wondering if i should delete the merged branch.
No, if you don't have disk space problem. Who knows what will be needed tomorrow.
It depends on what you created the branch for in the first place. A branch is usually used for maintaining a released or soon-to-be-released version of software, so in general, no, you would not delete it. Even if you do, it is still there taking up space (its just no longer visible in the current version), so unless you have a great need to delete it, I would keep it around.
I'd keep it. The SVN tree gives you a lot of history about your source code repository.
No,
if the moths are showing that your product was more stabil in the branch than in trunk, you may be want to start comparing both or starting the merge again.
Also customers could want to have additional features depending on the branch you had, because they do not want to risk that the applicatzion brakes, because of new code.
You should always have the possibility to revert changes. Thats the idear of branches, trunk and svn.
If you merge a branch into trunk using "svn merge --reintegrate", you are recommended to delete the branch. If you want to do further development in that branch, you should "re-branch", effectively create a new branch with the same name, but rooted at the same revision as you merged the branch into trunk.
Read more about svn merge --reintegrate.