The expression "survival of the fittest" doesn't mean what you think it means. It doesn't mean that the strongest, most aggressive survive. It means that species which are the best adapted to their environments will survive. 'Fit' is meant in terms of 'suitable'.
Furthermore, the theory of evolution by natural selection is descriptive not prescriptive, which is to say it tells you how something is not how it should be. Just because we can observe evolution by natural selection taking place, it doesn't therefore follow that we must approve of it. Think of it this way; we can understand how HIV works without believing that it is admirable or something to emulate.
The phrase 'Survival of the Fittest' didn't even come from Darwin - it comes from Herbert Spencer - though Darwin did later include it in a revised On The Origin of Species. We should also note that Darwin explictly states that to seek to remove the weak and helpless from society would be dehumanising and evil.
It's right to acknowledge that in some respects human beings have sidestepped the traditional evolution through things like immunisation (if we didn't have our childhood jabs, very many fewer of us would survive to adulthood) and medical technology, and there are certainly some who have claimed human evolution has ceased - (
http://scienceblogs.com/pharyngula/2008/10/ver y_peculiar.php) but it doesn't really make sense.
Firsty, it just changes the genetic diversity upon which natural selection can work and secondly, evolution takes a very long time. A lot of the advances in medical technology are far too recent for any effects to be seen.