What is the hidden meaning of 来打我呀?
来打我呀 (come hit me) is a taunt here.  Since the internet users in WeChat are not physically facing each others, it is impossible to reach over the monitor and hit somebody.  

Saying 来打我呀 (come hit me) remind the other person that "Yes, I am pissing you off, and there's nothing you can do about it."  ╮(╯ ∀ ╰)╭

You can taunt people on the web with anything that's impossible to do physically like 來咬我吃呀 (why don't you come bite and eat me), 來砍我呀 (why don't you come hack me up)