I don't think there's a right or wrong on this one.. in some cases, like child abuse/pornography then by viewing it you are condoning and helping sustain a market.. for many other things ... no not necessarily condoning it at all - might be just trying to get better informed.
I agree with Adaydreambeliever, but would add that in some countries - certainly in the UK - viewing paedophilic material is likely to lead you into very deep trouble with the law. I don't know if the same applies to terrorist and other crime-related sites.
I believe a lot of such material has been driven into the recesses of the "dark web" so even stumbling across it accidentally is fortunately very difficult or impossible for responsible Internet users.
You could though ask yourself whether you actually "need" to view any illegal or deeply immoral material, though I realise defining the latter where it's not actually illegal is a social minefield. Looking at it to help you understand the matter is not a defence in law if the material is illegal, because anyone can claim that (and some have tried it) and it raises why or whether you genuinely "needed to understand" it.
If access to any given web-site is by subscription then you purchasing it is definitely supporting it, but you could be counted as supporting it simply by free viewing, if only by adding to the site's "visit" count.
I would add that soon, in the UK you will have to prove your age to owners of pornography web-sites even if the contents are legal. How that will work, I do not know but I imagine a lot of users will stop looking at the sites rather than pass their bank details or other sensitive information to a site of dubious nature and unknown security.
