I think ....that serial killer can be yash's brother the one who shot his wife.... not sure if she was having affair or not actually .... but he thought she did and killed her.... , may projected random dead body to be him and disappeared...
and now killing all girls who he think is rebellious or is in love...he thinks...girls can NOT chose her partner/life...etc just like daadi...
.... that bride also was doing love marriage...
- he thinks he is punishing them... hence 'karma' tattoo
- he is sharper than police ...due to his experience
- he knows yash house very well ...can sneak in from anywhere...
- he thinks yash is cheating his fiancée as he saw him close to chikki