I believe that many inconsistent Calvinists worship the same God as I and sincerely believe He is loving.
And I think it can be argued that no Calvinist can be entirely consistent: nobody can truly desire to overcome sin in his life while knowing that God predetermined all his or her evil deeds.
And since the difference between single and double predestination is ultimately insignificant, this is the case for every Calvinist or Lutheran.
But people who really attempt to be consistent, like John Piper and McArthur (let alone Fred Phelps) inevitably turn God into an moral monster far worse than Staline.
I know many French and German racists who are almost infinitely more moral than the god of Calvinism.
Nearly consistent Calvinists adore an evil demon they call God.
That said I think it is important to always first and foremost see another person as a being created in the likeness of God.
I think it is important to withstand the temptation to become self-righteous and disagreeable.
When speaking with a Calvinist, I will first try to build a friendly relationship and avoid expressions which could uselessly hurt him or her.
I think it is important for her well-being that she gives up her Calvinist belief, but never at the expense of forgetting her humanity.
This is to my mind the main problem of many hardcore fundamentalists and of the New Atheists: while trying to win people for their ideas, they utterly forget love and spurn the most basic rules of human decency.
If we (both Arminian annihilationists and Universalists) act in the same way towards Calvinists, we are no better than hateful antitheists.
The only situation where I think it is permissible and just to use harsh words is when the individual repeatedly uses a hateful rhetoric, ridicule and mockery. It might be in order to confront him using a hard language, as our Lord Himself did with hateful pharisees.