adj. 25B6; noun
a malignant disease : VIRULENT, very infectious, invasive, uncontrollable, dangerous, deadly, fatal, life-threatening.
a malignant growth : CANCEROUS; technical metastatic.
a malignant thought : SPITEFUL, malicious, malevolent, evil-intentioned, vindictive, vengeful, malign, mean, nasty, hurtful, mischievous, wounding, cruel, unkind; informal bitchy, catty; poetic/literary malefic, maleficent.
benign, benevolent.