The English word "terrifying" has a very different meaning from "terrible/awful."
Terrifying = causing an extreme amount of fear Examples: "When my dog ran into the middle of a busy road, it was terrifying for me! Thank goodness he's okay!" or "The new ride at the theme park drops riders 200 feet straight down. It's terrifying!"
Terrible/Awful = Something that is extremely bad. The English word can refer to a situation or an object. Examples: "When my dog ran into the road and got hit by a car, it was terrible/awful!" or "This food tastes terrible/awful!"
The biggest difference between the terms is fear and an always-negative meaning. Terrifying always implies extreme fear, but while the situation causing that fear is usually bad, it can be good (like skydiving or a scary movie). Terrible or awful are always about something really bad, but they're most closely associated with the emotions of extreme dislike, sadness, etc. rather than pure fear.
A comma devides the sentence in sensible sections. Mostly to divide the standalone part from additional information. You can say "He is terrible. Period." but the "when he is jealous" is bonus information and cannot be standalone. English almost never uses commata but other languages like german do utilize to great extent.