Would having characters not feel sad about death, but instead fascinated about it be offensive to those who lost others? I always felt as if death is natural and should not be sad, but would writing it that way be offensive?
How one handles death and how one wants their own death to be handled are generally very personal and whether the rituals for the dead person are solemn, sacred and quiet or put the "fun" in "Funeral" is largely determined by what the deceased would have wanted.
For example, the funeral for Muppets creator Jim Henson was quite solemn and respectful... despite the fact that Big Bird showed up to eulogize Henson and sang "Bein' Green", the signature song of Kermit, the most famous character played by Henson. Other Muppet actors would also sing a melody of Henson's favorite songs in the voice of some of their characters (The puppets did not appear as unlike Big Bird, they were hand puppets and Muppets policy is that that the puppets and puppeteers are never to appear together).
Other memorial services are a bit zanier, such as the Monty Python's memorial for Graham Chapman, which started off with a bit of traditional low key Python humor (Singing of a traditial funeral him, but with fake Chinese accents) before going into the Eulogy.
John Cleese's first Eulogy starts off solemn... but in Python Tradition, it didn't end there:
I guess that we're all thinking how sad it is that a man of such talent, of such capability and kindness, of such unusual intelligence, should now, so suddenly, be spirited away at the age of only forty-eight, before he'd achieved many of the things of which he was capable, and before he'd had enough fun. Well, I feel that I should say, nonsense. Good riddance to him, the freeloading bastard, I hope he fries! And the reason I feel I should say this, is he would never forgive me if I didn't, if I threw away this glorious opportunity to shock you all on his behalf. Anything for him, but mindless good taste.
From there, the remaining service was respectful but silly and concluded with a singing of "Always Look on the Bright Side of Life" which found popularity with the British as it pokes musical fun at the nation's stoic nature in the face of danger and has since become the third most popular funeral song in the nation.
It should be pointed out that this service was for the public and while friends and family were invited and attended, the actual funeral was conducted two months prior and was quite private (The comedy troupe elected not to attend so as not to have the media circus surrounding his family's grieving).
I feel that it is difficult to predict what other people might or might not find offensive. Death is certainly a painful or frightening subject for many and I would consider it fair to warn readers of how you treat the topic in descriptions of your book such as the blurb or marketing summaries, if it is central to the story and you give it considerable space.
That said, there is no topic and no attitude that cannot be treated in literature. That is, in the opinion of the majority, the fundamental purpose of art. You can have psychopathic characters and discuss genocide. The question, when it comes to how readers and critics will judge your book, is not what you write about but how you write about it and what stance you, as the author, appear to hold.
In your case, there is a fundamental difference between claiming that death is (always) happy and that some people may experience death as happy under certain circumstances. The first is offensive to those who are afraid of death or have been hurt by it. The second is interesting and not offensive to anyone at all.
There will be characters who faint when they see a dead body and there will be those who say "is that a broken spleen? I might as well take a look!" and that's okay.
Characters deal with it depending on how they deal with life.
That's how it's supposed to work.
As long as you stay true to what your character would do, you're fine.
I can't see that being offensive at all.
I see I somewhat misinterpreted your question after seeing the answer from @fluctuating psychosis.
If you trying to show death as a "happy" event, then you may end up offending people. Just be careful. If you would be embarrassed if you showed it to a genocide victim, widow to a suicidal person, etc... then you may need to tweak something.
Books have glorified death in the past, some successful, some not. Consider leaving a disclaimer or author's note at the end of the book to explain any iffy decisions.