To foster a threat or be too lenient with a bad person, only to suffer later; to cherish a snake in one's bosom.

To tolerate or ignore a small problem or evil person rather than dealing with it early, leading to a much greater disaster later; literally "to foster an ulcer and bequeath a calamity"

giving in to evil or mistakes only creates more trouble for the future; to cause future disasters by being too lenient or indulgent