If only those two sentences actually appeared together in his letter. Which they didn't. You should actually try reading the source material instead of cherry picked, out of context, stitched together phrases.
Ben Gurion was speaking about the Negev in the first sentence, which he assumed would not be part of the Jewish state and would remain barren, unused because of lack of Arab competence, yet they would still prefer it vacant instead of allowing Jews to inhabit it. He did not wish to expel the Arabs and preferred they'd allow the Jews to use the land.
The second quote wasn't even in the letters to his son in 1937. It was written in his diary in 1947. It was a reference to how to respond to Arab attacks. The actual quote was:
"In every [Arab] attack, prepare to strike [in response] a decisive blow, destroying the place or expelling the inhabitants and capture their place.”
Given you just gobbled up the Ben Gurion quote without question you should expect to be ridiculed.
Can you explain some of Ben Gurion's other quotes, though?
"A partial Jewish State is not the end, but only the beginning. ... I am certain that
we will not be prevented from settling in the other parts of the country, either by mutual agreements with our Arab neighbors or by some other means. . . [If the Arabs refuse] we shall have to speak to them in another language. But we shall only have another language if we have a state."
As quoted in Chomsky, Noam, 1999.
In our political argument abroad, we minimize Arab opposition to us. But let us not ignore the truth among ourselves. I insist on the truth, not out of respect for scientific but political realities. The acknowledgement of this truth leads to inevitable and serious conclusions regarding our work in Palestine… let us not build on the hope the terrorist gangs will get tired. If some get tired, others will replace them.
A people which fights against the usurpation of its land will not tire so easily... it is easier for them to continue the war and not get tired than it is for us... The Palestinian Arabs are not alone. The Syrians are coming to help. From our point of view, they are strangers; in the point of law they are foreigners; but to the Arabs, they are not foreigners at all ... The centre of the war is in Palestine, but its dimensions are much wider.
When we say that the Arabs are the aggressors and we defend ourselves — this is only half the truth. As regards our security and life we defend ourselves and our moral and physical position is not bad. We can face the gangs... and were we allowed to mobilize all our forces we would have no doubts about the outcome... But the fighting is only one aspect of the conflict which is in its essence a political one. And politically we are the aggressors and they defend themselves. Militarily, it is we who are on the defensive who have the upper hand but in the political sphere they are superior. The land, the villages, the mountains, the roads are in their hands. The country is theirs, because they inhabit it, whereas we want to come here and settle down, and in their view we want to take away from them their country, while we are still outside. They defend bases which are theirs, which is easier than conquering new bases... let us not think that the terror is a result of Hitler's or Mussolini's propaganda — this helps but the source of opposition is there among the Arabs.
Address at the Mapai Political Committee (7 June 1938) as quoted in Flapan, Simha, 1979.
If I knew that it was possible to save all the children of Germany by transporting them to England, and only half by transferring them to the Land of Israel, I would choose the latter, for before us lies not only the numbers of these children but the historical reckoning of the people of Israel.
Attributed to Ben-Gurion (pre-War 1939) by Martin Gilbert in "Israel was everything" in The New York Times (21 June 1987)
Let me first tell you one thing: It doesn't matter what the world says about Israel; it doesn't matter what they say about us anywhere else.
The only thing that matters is that we can exist here on the land of our forefathers. And unless we show the Arabs that there is a high price to pay for murdering Jews, we won't survive.
As quoted by Ariel Sharon, in the documentary The 50 Years War : Israel & The Arabs (1999), this advice was given to him by Ben-Gurion after the controversial raid on Qibya.
[A quote about Ben-Gurion by Nahum Goldmann] One day, or rather night, in 1956 I sat up at his [Ben-Gurion's] house till three in the morning. That night, a beautiful summer night, we had a forthright discussion on the Arab problem. "I don't understand your optimism," Ben-Gurion declared. "Why should the Arabs make peace? If I were an Arab leader I would never make terms with Israel.
That is natural: we have taken their country. Sure, God promised it to us, but what does that matter to them? Our God is not theirs. We come from Israel, it's true, but two thousand years ago, and what is that to them? There has been anti-Semitism, the Nazis, Hitler, Auschwitz, but was that their fault? They only see one thing: we have come here and stolen their country. Why should they accept that? They may perhaps forget in one or two generations' time, but for the moment there is no chance. So it's simple: we have to stay strong and maintain a powerful army. Our whole policy is there. Otherwise the Arabs will wipe us out. I'll be seventy years old soon. Well, Nahum, if you asked me whether I shall die and be buried in a Jewish State I would tell you Yes; in ten years, fifteen years, I believe there will still be a Jewish State. But ask me whether my son Amos, who will be fifty at the end of this year, has a chance of dying and being buried in a Jewish State, and I would answer: fifty-fifty." "But how can you sleep with that prospect in mind and be PM of Israel too?" I responded. "Who says I sleep?" was Ben-Gurion's simple reply. That was Ben-Gurion all over: he had told me that so as to show me how well he knew in his heart that Israel could not exist without peace with the Arabs, but his stubborn, aggressive unbending character prevented him from following what his own intelligence told him. The best proof of that is that having lost his grip on power his intelligence reasserted itself; he even became a 'Goldmannite', declaring that all the occupied territories except Jerusalem should be restored. On this point I am in agreement with him: Israel must keep Jerusalem.
Nahum Goldmann, in The Jewish Paradox : A Personal Memoir of Historic Encounters that Shaped the Drama of Modern Jewry (1978)