Living creatures don't "decide" whether to be distasteful to eat - they evolve that way - if it gives them a reproductive advantage - and if their genetics give them the opportunity. Evolution is by chance and largely by natural selection. I don't know whether THC makes plants less likely to be eaten and hence makes the plants more likely to reproduce successfully. To answer that you'd have to study THC's synthetic pathway - the enzymes involved - related compounds synthesised on the pathway. However the advantage THC may confer on cannabis plants may not be due to being distasteful.
THC apparently absorbs UV strongly (or is an onti-oxidant, I'm not sure), so THC synthesis may have evolved because those precursor cannabis plants that happened to synthesise THC (or a similar related compound) were more resistant to UV damage. Again more study is needed.
Regarding evolution vs creationism - how would you explain flightless birds in New Zealand? You could say "God made them that way" or an evolutionist might propose that as New Zealand and related islands are extremely isolated, the first creatures to reach them were plants (seeds etc blown by the wind) and eventually birds arrived - again by air. New Zealand is volcanic, geologically isolated and relatively young - 5 million years or so. Once birds landed on the island they found there were no land living natural predators so with time new species of birds evolved that were flightless. The lack of predators meant that the selection pressure for flight was removed - allowing more evolutionary possibilities.
Man only arrived 2,000 years ago bringing rats and other mammals with them. The flightless birds were now at risk and most became extinct by predation.
I don't understand how you can defend a creationist argument when the evolutionary explanation is so much more powerful and testable.