From what I have heard here, they killed him off to fake out the audience so Lori's death would be a surprise.
What a shitty reason for getting rid of him - especially after they actually gave him some meaningful lines.
but tdog was already a side character the 1st 2 ssns. he looked like he was working out and ready for some lines this ssn and was immediately erased and for who?!! Lori. she is worse of a handicap but at least with the time skip between seasons we know that she can now at least use a revolver and knife