I think it's kind of funny that there are those in other churches who think of Adventists as "sheep-stealers," because such a large portion of those who become Adventists come from other churches. But I don't think you can blame an Evangelist for simply teaching Bible truth. If they want to be mad at someone, they'd have to be mad at God. For He is the author of the Bible and it's truths, not the Evangelists who proclaim it.
I heard a Pastor tell a story once that went something like this:
One day a Pastor from another church came up to me and said, "Stop giving Mr. and Mrs. Smith Bible studies! They are my members!"
To this I replied, "With all due respect, as long as they continue asking for Bible studies, I will give them."
This really made the other Pastor mad. He said, "Oh yeah? Well maybe I'll start giving Bible studies to your members!"
I just smiled and answered, "That's certainly OK with me."
So this Pastor started giving Bible studies to a couple from my congregation. A few weeks later I ran into him again and asked, "How are the Bible studies going?"
"We're not having the Bible studies anymore." He replied.
"Why not?" I inquired.
He answered sheepishly, "Your members know their Bibles too well."
And so it is, if we build our house on the Rock, though the rain falls and the wind blows, yet your house will stand.