Why did the father insist on looking for more people washed up by the storm? (chapter 2)

The Robinson family have been shipwrecked on a remote desert island. They find themselves in this predicament because their fellow passengers aboard ship evacuated without them, leaving the Robinsons to fend for themselves. Yet despite this, William Robinson sets out to see if he can find any other passengers from the ship that may have found their way to shore. His son Fritz doesn't understand why they should be out looking for them. After all, they had no qualms in abandoning the Robinsons to their fate, so why should they be helped?
But William replies by saying that searching for other survivors from the shipwreck is the right thing to do. The other passengers were wrong to abandon the Robinsons, but two wrongs don't make a right, or as William puts it, we should not return evil for evil. Moreover, if there are any survivors, then they're likely to be very hungry, and they can't just be left to starve. And in practical terms, the Robinsons need all the help they can get in building a shelter; the other passengers from the ship could be very useful to them in that regard.