Who said "We can come to look upon the deaths of our enemies with as much regret as we feel for those of our friends, namely, when we miss their existence as witnesses to our success."

I think you have been spending your day reading Forbes, mozart.
Of course, it was Arthur Schopenhauer who said this.

See this link to Forbes:

Ten things to do before you die: Make an enemy for life. A gentleman has been defined as someone who never gives pain to anyone unintentionally. This leaves plenty of leeway for deliberate rudeness toward someone you can't stand. Stand up to a bully; speak out against a fraud; fight off a rival for a prize. Care enough about something to make someone mad. An enemy helps you define yourself. As Schopenhauer said, "We can come to look upon the deaths of our enemies with as much regret as we feel for those of our friends, namely, when we miss their existence as witnesses to our success."

This advice goes against eveything I believe in...I can't think of anyone disliking me so much, that they would become my enemy, and I, theirs.

Yes that was an interesting afternoon. 10 things to do before you die