I wrote out this whole thing, but it just turned into a really, really long rant.
Basically, I think Mapleshade is quite happy where she is. I don't think she deserved to go to StarClan nor do I believe she would have liked it there. I think she probably would have crossed into the Dark Forest anyways. I think she's the most evil character in the series. If she gets to go to StarClan, what does that mean for cats that the majority of the fandom agree belongs in the Dark Forest? If Mapleshade belongs in StarClan, that means Thistleclaw, Tigerstar, Hawkfrost, Brokenstar, etc., belongs in StarClan too.
Also, nothing really excuses cold-blooded murder.