Why do insecure people feel the need to pick on people when they know what it's like to be picked on?

From what I can tell, it's because they want to regain the power taken from them. If they had previously been bullied, they might feel insecure and powerless. From their previous experiences, they only know of one way to take power, which just happens to be the same way they lost their own power.

People like that puck on people who cant defend themselves, so that they could feel strong and that they have power over this person, because they didnt have power on the people that picks on them, so they feel weak, thus they need someone to pick on to feel strong, and the cycle lasts forever

Because then the person they pick on has the same level of insecurity that they have and they feel equal and more powerful. It is sad really, but true. One has to be ready to manage this behaviour by treating it with silence. It is important to know the signs and watch out for this kind of behaviour and not to take it personally or seriously. Insecurity eats away at people and sometimes it stems from a bad experience that someone has had in their childhood or adult life. This makes them behave in this way. it is cause and effect.

Perhaps it is their way to feel powerful for once. Strangely enough we don't always learn from our own experience.