What are you going to do? Well, if you're a heavily armed, Houston cop, all alone (except for your partner), you draw your gun and plug him. Problem solved.
Houston police spokeswoman Jodi Silva said the man cornered the officer in his wheelchair and was making threats while trying to stab the officer with the pen.
At the time, the officer did not know what the metal object was that the man was waving, Ms Silva said.
"Fearing for his partner's safety and his own safety, he discharged his weapon," Ms Silva said.
Police did not immediately release the name of the man who was killed. They had been called to the home after a caretaker there called and reported that a man in a wheelchair was causing a disturbance.
The owner of the group home, John Garcia, told the Houston Chronicle that the man had a history of mental illness and had been living at the house about 18 months.
"He sometimes would go off a bit, but you just ignore it," Mr Garcia told the newspaper.
Wow, do the math - four arms, four legs, two guns, two truncheons, two Mace canisters versus one arm, one leg, a wheelchair and a pen.