This is where it's most important that the characters are balanced. It's not tragic unless you care for them. Set up earlier by writing/art/coloring leads up to this.
Writing Jim and Jennie - difficulty in balancing characters. This took a lot of rewriting.
Not make Jim too much of a prick so you can root for him after he gets the monster. He's motivated by money but he's not cruel. It's especially important that Jennie be completely sympathetic to balance out Jim. Write about edited character traits - drinking at party, etc. Not leave the location, either.