Essentially, the (French) text in the image attached to that tweet is saying that...
"Between Communism and Socialism there is the difference from assassination to imprudent homicide."In other words, they are both murderous ideologies. Now, it is significant that the writer died in 1966, so some of what was going on in places such as the USSR had become known to the wider world but much was still hidden. We certainly didn't have the awkward openness of the Internet that is so easily accessible by just about anyone in the free world these days.
Nowadays, and especially in the minds of those who caught the YouTube video of Stalin admitting to his party faithful that "Socialism is just the everyday word for Communism" before it was taken down, we know that there is essentially no difference in practice between the two. Socialism merely pretends not to be all-out murdering, thieving and oppressing Communism, using a wide range of propaganda and other techniques to camouflage its true nature.
This is the awkward (for them) twenty-first century, and we can now all see – if we pay even just a little attention – that there is no real difference at all. This is why self-styled Socialists tend to give the game away in unguarded moments, and we can quite easily see in due course that they are exactly the same as the Communists always were in outlook, attitude, and mindset in general.
That's just one reason of many why no truly decent person could ever be a 'Socialist', as they all support an ideology that (whichever way one looks at it) is murderous, among other undesirable and frankly downright nasty characteriistics. There are no exceptions to this!.