I think you're being awfully negative about this.
While I can see your point that you're not likely to ever influence the nature of human beings in any really remarkable way, this does not mean that you have nothing to contribute at all which might help 'perpetuate the existence of the human race'.

These computermanuals(?) that you write, could very well help people to master "computer networking" faster and better than they otherwise would have. That's something. It's really the way society works, everybody throwing in their two cents for the greater good.

As for the future of mankind, I remain cautiously hopeful. Really, I think we'll be alright. We have already come a long way, in a miniscule amount of time, technologically speaking. I do, however, agree that our obsession with killing each other might eventually get in the way of some accomplishments. Whether we'll outlive the sun... Well, that's anybody's guess isn't it? I mean... That big 'ol thing is going to be sticking around for a long (loooooong) time.

If you really want to get into it, you could discuss whether any action can really be called useful, on a long enough timescale? The invention of the steam engine is commonly regarded as one of the great scientific leaps in the history of mankind, but lately engines seem to be causing all the problems.(Global warming 'n such)
Computers are great, but if Science Fiction is right, they'll be ruling us before too long. One could probably find better examples than these, but you get my drift.(well, I hope you do)

And if you had saved Hitler from drowning, while he was still a child, would that have been an act of evil?