My senior coworker always stole credit for my work so I let him take the fall for my massive mistake

For two years my senior colleague Mark treated me like his personal ghostwriter. Every time I spent forty hours building financial models or pitch decks, he would swoop in at the last minute, slap his name in bold font on the title slide, and present my findings to the partners like he built the whole thing from scratch. Management loved him. I was just the quiet analyst in the corner. Three months ago I was running on four hours of sleep and made a serious calculation error in a revenue projection model for a major client. It threw off our quarterly staffing forecast by nearly eighty thousand dollars. when the regional vice president realized the numbers were completely detached from reality, he called an emergency meeting. He walked into the boardroom holding the printed slide deck, slammed it on the table, and stared directly at Mark. The title slide said "Lead Strategy and Financial Audit: Mark Henderson." The vice president asked him how on earth he could sign off on such an amateur screw-up. Mark panicked. He turned bright red, started sweating through his dress shirt, and tried to stammer that I was the one who pulled the raw data. The vice president cut him off immediately and said, "Your name is on the cover, Mark. You told us you verified every cell." I sat right next to him. I could of raised my hand and admitted it was my data entry error. Instead, I stared blankly at my legal pad and said NOTHING. Mark lost his quarterly bonus and got demoted to internal accounts. its the dirtiest thing I have ever done, but I have never slept better in my life. Am I a monster for this?