This is the only place in the Pauline letters that directly accuses "the Jews" of killing Jesus. I'd long just thought of this as some later interpolation, but it is indeed attested to in Marcion's version of the letter.
14 For you, brothers and sisters, became imitators of God’s churches in Judea, which are in Christ Jesus: You suffered from your own people the same things those churches suffered from the Jews 15 who killed the Lord Jesus and the prophets and also drove us out.
From Tertullian, Against Marcion 5.15
I shall not be sorry to bestow attention on the shorter epistles also. Even in brief works there is much pungency. The Jews had slain their prophets. 1 Thessalonians 2:15 I may ask, What has this to do with the apostle of the rival god, one so amiable withal, who could hardly be said to condemn even the failings of his own people; and who, moreover, has himself some hand in making away with the same prophets whom he is destroying? What injury did Israel commit against him in slaying those whom he too has reprobated, since he was the first to pass a hostile sentence on them? But Israel sinned against their own God. He upbraided their iniquity to whom the injured God pertains; and certainly he is anything but the adversary of the injured Deity. Else he would not have burdened them with the charge of killing even the Lord, in the words, Who both killed the Lord Jesus and their own prophets, although (the pronoun) their own be an addition of the heretics. Now, what was there so very acrimonious in their killing Christ the proclaimer of the new god, after they had put to death also the prophets of their own god? The fact, however, of their having slain the Lord and His servants, is put as a case of climax. Now, if it were the Christ of one god and the prophets of another god whom they slew, he would certainly have placed the impious crimes on the same level, instead of mentioning them in the way of a climax; but they did not admit of being put on the same level: the climax, therefore, was only possible by the sin having been in fact committed against one and the same Lord in the two respective circumstances. To one and the same Lord, then, belonged Christ and the prophets.
A few things:
1) This indicates that the "churches" in Judea were Gentile.
2) There are propositions that 1 Thess 2:13-16 are an interpolation. But if that is the case, it was interpolated before prior to Marcion.
This raises some complicated issues. Either:
1) This is all authentically from Paul, and Paul believed that Jesus was executed by the Jews, but he never made that clear anywhere else.
2) 1 Thess is a forgery in its entirety (however it is used by the writer of Mark also)
3) 1 Thess 2:13-16 is an interpolation, added by Marcion himself. That it exists in the orthodox version then can only be explained by the orthodox version having derived from Marcion's version.
4) 1 Thess 2:13-16 is an interpolation added to the base sources, prior to Marcion. The orthodox version may come from an independent branch of the documents.
5) 1 Thess 2:13-16 contains some post-Marcion interpolated revisions, but the core is original. (see issue #1)
None of these seem satisfactory. This seems to be a really problematic passage. Thoughts on it?