Oh, I can be creative.
Variant 1: we exhaust so many resources on our planet that our space program becomes stagnated, we fail to develop an alternative to chemical rockets, and we are left stuck in our own pot, limited to launching slow scout probes to crawl through space that will send back data about something useful after several millions of years, if we're lucky. Meanwhile we're busy having wars, having peace, and reproducing until we run out of space. No significant death count - it's a slow, boring end.
Variant 2: found deep underneath the Sahara desert is an enormous, ancient, advanced device of unknown purpose. Years are spent investigating it, cults are formed around it, conspiracy theories are exchanged, until one day, without any explainable reason, the device detonates with inconceivable force, causing not only the immediate death of almost all life on earth, but also pushing the planet off orbit. Somebody set us up the bomb. Before we were even born.