The people that illegally sneak over the border are people who are more likely to do crimes once here. They already broke the law by illegally breaking into the country, showing their propensity to engage in illegal acts. And statistics do show that once here, the illegal aliens are far more likely to be criminals (meaning additional crimes than illegally entering the country) than immigrants who immigrate legally as well as American citizens.
Trump spoke the truth too loud so the left went wild.