Just look at the two most crucial issues: climate change and nuclear proliferation. Trump doesn't even believe global warming exists, he has claimed it's a conspiracy invented by the Chinese.
And, in a briefing, he asked three times why we don't use nukes if we have them. It was explained over and over to him, and he just couldn't grasp why we don't use nukes. He's extremely thin skinned, and the evidence of that is so overwhelming we shouldn't even need to discuss it - it's uncontroversial. Having a guy like trump have access to the nuclear codes is a recipe for disastor. He could launch a nuclear strike over a mean tweet at 3:00 am - and that's not an exaggeration, there is almost nothing stopping a president from immediately launching a nuke. He's also said that he wants more countries to have nukes, like south korea. And just to show how little it would take for him to start a war, which could lead to nuclear war, he said, in response to Iranians giving the middle finger to American ships, that, if he was president, he would've had the Iranians blasted out of the water. This, of course, would start a war. He would start a war over hand gestures.
Then, of course, you have the fact that he's a sexual predator, that he has the vocabularly of a third grader (literally, it's been analyzed), that he doesn't want to help the refugees, wants to ban muslims and have racial profiling, wants to build an expensive and impractical and nearly impossible wall because he's racist against mexicans, would order war crimes to be committed (he wants to target the families of terrorists - a war crime), he wants to have the largest tax cuts in history for the wealthiest top 1%, and he wants to deport millions of immigrants.
Clinton is a horrible choice, but it's not even close btwn her and Trump. Trump is the most dangerous candidate we've ever had.