The man is unprincipled and immoral, and there are hundreds of examples that he has left in his wake to prove the fact that he stands for nothing other than his own self-aggrandizement and wealth/power accumulation. He has stepped on, and mercilessly slandered, more people (both 'little people' and big ones) on his way to the top than most of us even realize.
The thought of a man as personally principled and firmly rooted in the Constitution as Ted Cruz (the only candidate fully capable of filling Ronald Reagan's shoes who has come along since Reagan left the White House) teaming up with such an unprincipled ogre is unconscionable.
We are not supposed to compromise with evil, not matter the threats or repercussions involved.
Even if all we knew about Donald Trump were his most recent offenses committed against Heidi Cruz, that would be sufficient to disqualify him from holding America's highest office, on grounds that he is a lying, superficial, petulant character assassin.
Unfortunately, he has committed significantly more, and deeper, immoral crimes than this latest one. The count increases daily, and if that fact is not significant enough for the American electorate to turn away from this evil man, then America, unfortunately, deserves the bleak future that will lie ahead, whether it be under a Trump or Clinton presidency. If you place your faith in an unprincipled, characterless candidate (no matter the capital letter behind his/her name) who has no respect for truth or personal integrity, you are rolling a pair of dice whose outcome will be disastrous, at best. History is replete with such examples, but we Americans appear to be slow learners, despite the brilliant, painstaking roadmap laid out for us by our Founders and the unimaginable sacrifices they made in order to author it.