At the end of the day, you're using Slayer.  I've found that, with few exceptions, the bundled generator plugins are pretty weak.  On top of that, a guitar is a very difficult instrument to emulate.  Slayer is pathetic for the most part.  If, by some incredible chance you manage to get a good sound out of it, play a R5 chord and it goes muddy.

If you have no access to a guitar, then the best you can do is use Slayer, try output the cleanest tone, and route that through a VST made for guitars.

But use a real guitar if you can.