Six weeks after a white supremacist gunned down Pardeep Kaleka’s father and five others at a Sikh temple in the U.S. exactly a year ago, Mr. Kaleka was skeptical when a former skinhead invited him to dinner.

But Mr. Kaleka accepted, and he’s grateful he did. Since then, the grieving son and repentant racist have formed an unlikely alliance, teaming up to preach a message of peace.

They’ve grown so close that they got matching tattoos on their palms the numbers 8-5-12, the date the gunman opened fire at a Sikh temple in Wisconsin before killing himself.

It wasn’t easy for Mr. Kaleka to meet Arno Michaelis (42), who admits he contributed so heavily to the white-power movement that he might have influenced the shooter. Mr. Kaleka knows Mr. Michaelis’ history — his lead singing in a white supremacist band; the white-power and swastika tattoos; the countless fights; and more than a dozen arrests.

But he also saw the good work Mr. Michaelis has done since he quit the racist movement in the mid-1990s. Mr. Kaleka (37), wanted his father’s death to be a catalyst for peace, and saw in Mr. Michaelis a partner whose story could reinforce the message that it’s possible to turn hate into love.

Mr. Michaelis had written a book called My Life After Hate, in which he describes how he lashed out at the world and how the birth of his daughter made him realise he needed to change.

The two men have teamed up to create Serve2Unite, a community group that works to counter violence with peace. Mr. Kaleka, Mr. Michaelis and others visit schools, where Mr. Kaleka describes how gunman Wade Michael Page walked into the Sikh Temple of Wisconsin and killed six people he didn’t know. Then Mr. Michaelis describes how the gunman’s white-supremacist background was nearly identical to his own.

When kids hear from someone who used to be a violent hater, and then from someone whose father fell victim to that hatred, the message is sobering.

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