Shankara did believe in the equality of the five gods this is also clearly mentioned in the Yajurveda sandhyavandam and the Atharva Veda as well as Yajur and Rigveda.
From Yajurveda sandhyavndam.
namaH savitre jagadeka chakShuShe jagat prasUti sthitinAsha hetave| viri~nchi nArAyaNa shaMkarAtmane
In this verse the quality of Shankara and Narayan is ...
adiyEn rAmAnuja dAsan
I have a very simple explanation from learning thiruvAimozhi and looking up meanings at a very superficial level.
nammAzhwAr in thiruvAimozhi; considered to be drAvida vEdam says in 1.1.5
avaravar thamadhamadhu aRivaRi vagai vagai avaravar iRaiyavar enavadi
adaivargaL avaravar iRaiyavar kuRaivilar iRaiyavar avaravar vidhi
Why is killing a brahmin considered sin while killing other caste people is not?
This presupposition is wrong; killing people of other castes is also a sin:
Manusmriti 11.126: One fourth of the expiation for the killing of a Brāhmaṇa has been prescribed for the killing of a Kṣatriya, one
eighth for that of a Vaiśya, and one sixteenth for that of a