There are so many factors:
1. What kind of mood the poster is in when they access the forum;
2. Whether the subject matter is "near and dear" to the poster's heart;
3. What type of person the poster is in real life (many are just as obnoxiously confrontational in real life);
4. The poster is a cyber-bully who lives for confrontation;
5. Whether they see the forum as "entertainment" or "informational" (if the former, they usually try to spice things up);
6. There is a history between the OP and the "snarky" poster;
7. The OP deserves the snarkiness for one reason or another.