Did you also run the specific McAfee removal tool?
If not please do so now, see:
http://service.mcafee.com/FAQDocument.aspx?id=TS101331
direct download: http://download.mcafee.com/products/licensed/cust\_support\_patches/MCPR.exe
After a reboot, Windows Defender should be automatically enabled.
If not, please try everything here: Problems starting Windows Defender in Windows 8/8.1/10
Cheers,
J