These lines can be read a number of ways:

1. Their revenge is like a fever which burns in them, a dire disease (dear cause) moving even the calmest (mortified) among them to shed their own blood in battle. “Bleeding,” the practice of cutting a patient to let blood out, was thought to cure a fever.

2. Their cause is so honorable (dear) that even an insensitive (mortified) man would respond to (be excited by) their call to arms (alarm).

3. Their motivation (cause) for killing Macbeth was intense (dear); the wounds of their dead (mortified) compatriots had opened back up (been excited to bleeding). It was believed that a murder victim’s wounds bled in the presence of the murderer.