If he wanted to protect poor Poland from the evil Germanz, surely telling the world about Hitler’s evil plans would have been even better? Or at least warning Poland.

This assumes that 1) Poland was somehow unaware of what its neighbor was up to and 2) the USSR was aware and chose not to tell them. Those are some big assumptions.

  On March 17 [1939] the British Government suddenly remembered the existence of the Soviet Union and inquired what its attitude would be toward the Hitler threat in Eastern Europe. Moscow replied promptly and proposed an immediate conference between Britain, France, the U.S.S.R., Poland, Rumania and Turkey to consider how to resist German aggression. This was exactly and obviously what was urgently needed. Nothing less than a drawing together of all the threatened states could be of any avail. Rumania was under intense pressure to turn over her economy to Germany and the quickest action was needed.
  However, the Chamberlain clique could not make the shift. Knowing that they wanted to solve the Nazi menace on the plains of Russia, they ascribed to Russia the very same design of which they were guilty, as devious men so often do. They had a “deep seated conviction,” shared also by the French Rightists, that Russia wished to destroy the capitalist system in Europe by provoking a war from which she would remain aloof.5 The entire diplomatic record of the past five years belied this self-justifying suspicion. It showed that Russia was desperately anxious to avoid war, but also that on every occasion, without exception, she had sought to avoid war for herself by combining with others to prevent aggression or nip it in the bud. It was Russia which had incessantly pleaded that “peace is indivisible,” warning that if war came all would be engulfed in it.
  All this had meant nothing to conservative men bent on making terms with fascism and preserving it. Now, therefore, Chamberlain hesitated a week until Rumania capitulated and on March 18 notified Russia that her conference proposal was “premature.” This was the same rebuff given to Russia when she had proposed a conference a year earlier, at the time of Hitler’s conquest of Austria. Again there was no hurry, but this time Chamberlain did propose a substitute plan whereby Britain, France, Russia and Poland would consult if any further acts of aggression were believed to be imminent, but even this proposal was abortive, since it was at once learned, says Chamberlain’s biographer, that “Poland would refuse contact with the Soviet, which alone was enough to prevent us from taking up the Russian proposal for a six-power conference.” Chamberlain did not blame the Polish Government. He confided to his diary, on March 26: “I must confess to the most profound distrust of Russia,”6

The Cold War And Its Origins, 1917–1960. Vol. I, 1917–1950 By Denna Frank Fleming, Chapter 5

The classic combination of being extremely smug while also being factually wrong

He literally did try to warn about the fascist threat repeatedly and was constantly rebuked by the west due to anti-communism and "appeasement" politics.

They also weren't in a position to go on an offensive against Germany all alone if that's what you're gonna suggest next.

Their only option was a deal along these lines to buy time for themselves to build up productive forces.

You really think no one knew what Nazi Germany was up to and that it should have been the USSR's job to tell the world? lmao