Some years back in Mumbai there were gang wars mostly between different gangs (Chota Shakeel gang, etc). But I think killing has reduced.
But there has not been target killing where political parties are attacking people of other party affiliation.
Mumbai is on the same scale as Karachi 20 million, but if something like this happens it is very hard to control. Eg. Riots if they happen are hard to control in Mumbai because people can take advantage in Riots by killings enemies.
Not sure about Delhi crime. But i suspect , Mumbai is much better than Delhi