Kittel was the editor of the TDNT which is still a primary resource in biblical studies (it is commonly bundled in the Logos platform). But the Nazi background of the work makes its use problematic. There was an interesting article about this by Maurice Casey (NT, 1999) documenting an antisemitic Tendenz to the German version of the TDNT, as well as the Nazi activities of its contributors. Let me quote one passage:
To understand this, we must return to Nazi Germany. While scholars are not allowed to read Grundmann's personal files at church archives until 2006, a quite overt example of the prevention of knowledge, we know enough to put his TWNT article into its cultural context. Grundmann joined the Nazi party on 1st Dec, 1930 (membership no. 382 544), and became active in the Deutsche Christen movement. He served as Kittel’s assistant from 1930-32, preparing TWNT, to which in due course he contributed several articles. In 1932, he received his doctorate from Kittel at Tübingen. On 1st April, 1934, he became a supporting member (Förderndes Mitglied) of the SS (Membership no. 1032691). In 1936, he became a professor at Jena. He had not written a Habilitationschrift, but Hitler signed his appointment following a recommendation in which it was said that the Faculty wanted to become a stronghold of National Socialism, so that Grundmann’s scholarship could be path-breaking for a National Socialist perspective in the realm of theology. The year after the publication of his article on καρτερεω etc. was the year of the opening of the Institut zur Erforschung und Beseitigung des jüdischen Einusses auf das deutsche kirchliche Leben (Institute for the Study and Eradication of Jewish Influence on German Church Life), with Grundmann as its academic director. His address at the opening, on 6th May, 1939, was programmatic: "The Dejudaization of the Religious Life as the Task of German Theology and Church". This declared that the elimination of Jewish influence on German life was an urgent task. There should therefore be no doubt about Grundmann’s central life-stance. He was not a frightened rabbit, nor someone doing his best in more difficult circumstances than we have to live through: he was a committed anti-semitic Nazi.