I mean, what's the purpose of those virus? - mikeflood
A virus has no purpose. Arguably it isn't even 'life', it's just a chunk of DNA (actually RNA in the case of ebola) that gets replicated by hijacking the cellular machinery of its host.
Just to be killing humans for the sake of killing?
It lives harmlessly in fruit bats. It transfers to humans through bushmeat after which it becomes lethal and passes on through bodily fluids.
Could be an evolutionary purpose?
Human DNA has evolved to maximise its chances of being passed to future generations. But like every organism so has virus DNA. Unfortunately the relationship between our genomes is not symbiotic. Virus DNA/RNA does not intend to kill its host but neither does it care. From the perspective of ebola human death is collateral damage.
The risk of a global epidemic is real.