Earlier quoted context omitted.
It was a professional hit job given the description here...the killer was wearing gloves, a mask, had an empty suitcase and rode the elevator with the victim to his apartment. The hitman was likely doing a deed for someone who paid for his service...
Not sure why everybody is suggesting a professional hitman based on the fact the murderer used a mask, gloves and a suitcase. I suppose you get that idea after watching an arbitrary crime movie. All it proves is intention.
The other clue is from the articles I've read is that there was no blood. No average person with a bone saw is going to be able to pull that off. Sorry.