Depends on your jurisdiction. In Germany, the bar for murder is one of [1]:

• an [inner] desire to kill
• killing for sexual reasons
• greed
• “other despicable motivations”

and one of:

• cruelty
• insidiousness
• using “tools dangerous for public safety”
• the motivation to conceal or enable another crime.

In my layman’s understanding, pressing the button checks the ‘insidious’ box, since it is not at all expectable by the victim.

As for the first set of conditions, it would probably be down to a court ruling if the person checked the ‘desire to kill’ box (as I understand it, they wouldn’t) or the ‘other despicable motivations’ box – is killing for no reason a despicable reason?

[1] www.gesetze-im-internet.de/stgb/__211.html