I say...
Just think of it like this...
From the SRD:
Good vs. Evil
Good characters and creatures protect innocent life. Evil characters and creatures debase or destroy innocent life, whether for fun or profit.
"Good" implies altruism, respect for life, and a concern for the dignity of sentient beings. Good characters make personal sacrifices to help others.
"Evil" implies hurting, oppressing, and killing others. Some evil creatures simply have no compassion for others and kill without qualms if doing so is convenient. Others actively pursue evil, killing for sport or out of duty to some evil deity or master.
"Neutral" people are committed to others by personal relationships. A neutral person may sacrifice himself to protect his family or even his homeland, but he would not do so for strangers who are not related to him. People who are neutral with respect to good and evil have compunctions against killing the innocent but lack the commitment to make sacrifices to protect or help others.
Animals and other creatures incapable of moral action are neutral rather than good or evil.
Helm's DOGMA: (from Forgoten Realms setting pg 242)
Never betray your trust. Be vigilant. Stand, wait and watch carefully. Be fair and dilligent in the conduct of your orders. Protect the weak, poor, injured, and young do not sacrifice them for others or yourself. Anticipate attacks and be ready. Know your foes. Care for your weapons so they perform their duties when called upon. Careful planning always defeats rushed actions in the end. Always obey orders, providing those orders follow the dictates of Helm. Demonstrate excellence and purity of loyalty in your role as guardian and protector.
OK...being evil here is gona be tough. The way I'd play it is that the cleric has had an epiphany or experiance that leads him to be very militant in the spread of Helm's dogma. This could be played in line with the tyranny aspect of the LE alignment. I mean the worsip of Helm is the only true faith....join us or die. About the protecting the inocent crap, that applies ONLY to the worshipers of Helm...the rest are not inocent if they don't believe in the salvation of Helm's worship.
Likewise, putting the poor and injured to the sword is protecting them in a way. I.E. Commoner to Cleric - "I can't feed my family" Cleric ending the guys life - "OK now there is one less mouth to feed = mercy and protection. I end the suffering ot the injured so that they may be jusdged by Helm for thier life and rewarded/punished accordingly = I have ended their pain and am thier guardian. I mean after all LE people have the view that most people (other than themselves) are weak, incompetent, and need to be "shown the light".
Others are the sheep and the cleric is the shepard, and the shepard has responsibility to guard and protect the flock right.... That often means culling the weak from the flock to ensure that the only healthiest have an increased chance of thriving.
Also the flock is there for the shepard to exploit not vice a versa, he can sell, exploit use and even eat them (I wouldn't go this far wiht my flock of people, lest I be hunted down) as he needs them.
This is converse to the traditional role where the cleric is there to serve the people. I'd go crazy with this aspect...these sheep are here for me...(extract brutal and heavy tithes, even at sword point) I need gold to carry out Helms work. They complain becasue they are weak of faith and do not understand their role in Helm's plan like I do.... I put them down to make them strong....Give till it hurts!!! All for Helm!!!
Anyway, just make him more militant and brutal in his role as the shepard. Have fun with it. I know I would. I love evil PC's.