Why are jews called christ killerss?

So I started attending shabbat services, and the rabbis bible class like every week I'm 23 yrs old I def feel outta place I feel like I'm always the young person there and I say whu am I here, but obv its my faith why I go. Anyways I over heard the rabbi say yeah like nott in a serious way but he said people call us christ killers! In my head I'm like why?