In act 1 of Arsenic and Old Lace, Mortimer's sociopathic brother, Jonathan Brewster, arrives. He startles his aunts, who do not recognize him because he has had surgeries on his face to disguise himself. The surgeon who altered his face the last time is named Dr. Einstein. He is an alcoholic. He was drunk and watching a horror film starring Boris Karloff when he did the surgery on Jonathan. 

When Jonathan arrives at his aunt's house, his two aunts do not recognize him. Jonathan tells them that he has had surgeries on his face done by Dr. Einstein. He then proves his identity and tells his aunts that he and Einstein are in business. After the aunts leave the room, Dr. Einstein tells Jonathan that since they are wanted for murder, they need to decide what to do with the body of a man named Spenalzo that is out in their car. Jonathan admits that he killed this Mr. Spenalzo because he made the remark that Jonathan now resembles Boris Karloff after Einstein's surgery. However, the man's remark was appropriate since Jonathan's murderous act was monstrous. It is ironic that he feels insulted by this remark because he is as sinister as the characters that Karloff portrays.

Jonathan looks like the famous horror actor Boris Karloff because he has had plastic surgery to disguise his appearance as he is wanted by the police.  His surgeon was drunk while performing surgery and had Karloff on his mind while working.

When people mention the resemblance, Jonathan becomes enraged that he would be compared to someone who is known for portraying monsters in the movies.

The irony, of course, is that Jonathan is a murderer many times over, and is, therefore, a monster himself.  He is such a psychopath, he has no clue that he is such a fiend, but feels his actions were warranted with each killing, and plans to kill his brother Mortimer as well.