the "gear" thing down in the dock
Go to Accounts or Users & Groups (whichever one you have)
Single click on your account in the left hand pane
click the padlock, if necessary, to unlock the panel
In the right hand window, still with your account selected, click on Login Items
If there is anything in that list that relates to MacKeeper, click it to highlight it and click the "-" button
That will remove it from the list.
Repeat as necessary.
On completion, close the window and go to the Apple menu at the top left of the screen.
Choose Log Out
Once you have logged out, and you see the login screen, log back in again.
MacKeeper should now allow you to delete it.
If not, restart and try to delete it.