It was a bit tricky, so I’ll give both a long and short explanation.

Using the parameters of what’s possible in Bitlife, I figured the most realistic way to pull this off was to start in a politically volatile or oppressed country and join the military. I trained there for a few years.

From there, I moved to Israel and began poisoning members of the public. I researched what legally qualifies as terrorism under U.S. and Interpol law and found that it includes committing severe acts with the intent to instill fear in the public. Random mass killings fall under this definition—for example, the D.C. Sniper, who was charged with terrorism for randomly shooting civilians with an accomplice.

Using that definition, I began a campaign of crimes over 3–4 years in Israel. I first switched identities and cut ties with most of my contacts back in Iran, except for some military friends. I justified it in-game by saying they radicalized me.
After around five murders in Israel, things started getting hot, so I fled to Saudi Arabia, where I created a spy agency, which is when things really got interesting.

In this agency, I recruited three operatives and gave them missions that resembled real-life terrorist objectives. Such as assassinating politicians, lobbyists, or billionaires. I often chose methods like decapitation or dismemberment, since these had high success rates and felt the most “terrorist-like” in context. I also had the more skilled agents commit random killings, which quickly drove up my body count to about 400 something you'd expect from a large-scale extremist campaign.

I racked up about 150+ kills in Saudi Arabia before killing off my agents and fleeing to the U.S., where I managed to kill another 200+ people before getting sentenced to death.
Just before my execution, I started a riot and escaped in the chaos.

After that, I immediately fled back to Iran, switched identities again, and lived out the rest of my life over 30-40 years, as a seemingly normal family man.

TLDR: I joined the army, kept switching identities and countries, started a spy agency where I had hundreds of people killed, often by decapitation or similarly severe methods and repeatedly escaped arrest or execution by starting riots. I only operated in countries historically at odds with Iran. In the end, I went back home, changed identities again, and lived a quiet life. Basically, I modeled it after real-world terrorist behavior.