When Mattathias saw it may be rendered “When Mattathias saw what was happening [or, what the man was doing].”

He burned with zeal is literally “and he was zealous,” which means he was overcome with patriotic and religious passion. This clause may be rendered “he knew that he had to do something.”

And his heart was stirred is literally “and his kidneys trembled.” In a number of languages a similar idiom will be suitable here. An alternative model for this clause and the previous one is “his feelings were so intense that he was shaking.”

He gave vent to righteous anger: Righteous anger is “appropriate anger,” “anger that was justified.” Good News Bible translates this clause as “he became angry enough to do what had to be done.” Another model is “he became very furious at this wicked deed.”

He ran and killed him upon the altar may be expressed as “he ran over and killed the man right there on the altar.”

An alternative model for this verse is:

• Mattathias knew that he had to do something when he saw what was happening [or, what the man was doing]. His feelings were so intense that he was shaking. He became so furious at this wicked deed that he ran over and killed the man [right there] on the altar.

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on 1-2 Maccabees. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here.