malignant

Tending to metastasize.

Virulent or threatening to life.

Having or showing ill will; malicious.

Disposed to inflict suffering or cause distress; having extreme malevolence or enmity; virulently hostile; malicious: as, a malignant heart.

Virulently harmful or mischievous; threatening great danger; pernicious in influence or effect.

Specifically

In astrology, threatening to fortune or life; fateful: as, the malignant aspect of the stars.

In pathology, virulent; tending; to produce death; threatening a fatal issue: as, a malignant ulcer; a malignant fever; malignant pustule or scarlet fever.

Extremely heinous: as, the malignant nature of sin.