ClearAll["Global'*"]

<<Feyncalc`

(*Compton scattering/Setting momentum values*)
SP[p1,p1]=SP[p4,p4]=0
SP[p2,p2]=SP[p3,p3]=m^2
SP[p1,p2]=SP[p3,p4]=s/2-m^2/2
SP[p1,p3]=SP[p2,p4]=m^2/2-t/2
SP[p1,p4]=SP[p2,p3]=-u/2

compa=e^4/(s-m^2)^2Tr[GA[μ].(GS[p1]+GS[p2]+m).GA[ν].(GS[p3]+m).GA[ν].(GS[p1]+GS[p2]+m).GA[μ].(GS[p2]+m)]//Contract//Simplify

compb=e^4/(t-m^2)^2 Tr[GA[ν].(GS[p1]-GS[p3]+m).GA[μ].(GS[p3]+m).GA[μ].(GS[p1]-GS[p3]+m).GA[ν].(GS[p2]+m)]//Contract//Simplify

compab=(2*e^4)/((s-m^2)(t-m^2))Tr[GA[μ].(GS[p1]+GS[p2]+m).GA[ν].(GS[p3]+m).GA[μ].(GS[p1]-GS[p3]+m).GA[ν].(GS[p2]+m)]//Contract//Simplify

comp=compa+compb+compab//Simplify

comp/.u->2m-s-t//Simplify

comp/.m->0