A quick word of warning: not only will this not work, but in fact it will do nothing but earn you a polite lecture on how their beliefs are almost exactly the opposite of what you have just said.
They will be particularly quick to point out to you that their vision of Armageddon is not in fact the end of the world, as this is a common misconception about Witnesses. People will die, but even this will not be permanent (at this point they will also probably bring up the fact that they don't believe in Hell). Then they might point out to you the humanitarian efforts in which they assist; they don't get much attention for this because they don't spend incredible amounts of money loudly advertising it.
The wrap-up will probably consist of the assertion that their vision of the Good News is exactly as you pointed out: that God loves us and cares about us. Then, most likely, they'll politely leave, satisfied that they did exactly what they came to your door to do: to teach you something about their religion, not necessarily to convert you (that being your decision to make, not theirs).