I believe they have infinite lifespan since they were made to keep Mata Nui alive and things like that.
But, besides the fact that Greg confirmed they do have maximum lifespans, Mata Nui can be kept maintained by new Matoran replacing old ones, similar to the current real-life system for humans. 
(And since the Red Star stopped teleporting people back, that's what's been in place in the cases of accidents and murders and the like.)
Murderer wouldn't figure into the picture. The Matoran weren't supposed to be sentient
Not sure what you mean; what picture? I only mentioned murders because they do happen and that's part of what new Matoran replace now that the RS is broken. Even if they didn't, there are still deadly accidents, so it doesn't matter.
But since you bring it up; "murder" would be possible even without sentience if a unit glitched to start "demolishing" fellow units. In fact IMO nearly anything bad that a sapient evil being can do, the GBs could also consider as a possibility for a glitching AI.
Some animals can live forever on Earth I believe. No reason my a Bionicle might not be able to
No animals but the trees sequoya sempervirens and the taxus can.
No lifeform at all lives forever in the current system. Even communal organisms like coral (not sure if that's accurate that it applies to any trees BTW) will eventually die out due to accumulating mutations. They live beyond individuals because the individuals are relatively small. Technically communities of animals, and humans, live similarly, so it's really just a play on words. Individuals die of old age mainly because (as far as we know right now) the "bookends" if you will on our DNA strands wears down over time, and when those disappear completely our DNA itself starts to unravel. New individuals get full-length bookends.
So, if there was a biological repair system for the bookends (as we may technologically accomplish in the future), our lifespans could be extended much longer. Just making longer bookends at the beginning would also lengthen them, but constant repair could enable us to live liflespans more similar to Agori.
So my guess is that Agori have such a biological system.
What would, then, kill off an individual Agori, if so, would be other deleterious mutations to their DNA (equivalent, if they don't use DNA), but other repair mechanisms could extend even this, until finally the wear and tear on the repair mechanisms themselves would spell the end.
We're missing a very inportant key element here, we don't know how long a year in the Bionicle universe is.
No, Greg confirmed a long time ago it's Earth years. What we don't know is how long the days are compared to us and thus how many of them there are in a year. But it's probably essentially the same IMO, with each day split up into more and shorter hours.
The trees can regrow themselves when the are close to dieing(not counet cutting or lightning or forestfires) and the year stuff I remembered reading something in the first ask Greg stuff he wasn't shure how long a Bio year took but that could've changed over the years
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