Adam Clarke on Psalms 145:20

The Lord preserveth - He is the keeper of all them that love him.

But all the wicked will he destroy - They call not upon him; they fight against him, and he will confound and destroy them. There is something curious in the שומר shomer, the keeper or guardian of the pious; he is שמיד shamid, the destroyer of the wicked. The first word implies he is continually keeping them; the second, that he causes the others to be destroyed.