The gemara in Megila 7b says קם רבה שחטיה לרבי זירא Rabbah got up and killed Rabi Zeira. Why did he need to get up? Rabi Zeira was a short man, as we see in the bottom of Bava Metzia 85a קטין חריך שקי, and Rashi explains דאיניש גוצא הוה.
Because the Aruch HaShulchan (YD 24:13) rules to slaughter while standing rather than while sitting:
אין לשחוט מיושב אלא מעומד
Chazal tell us that
כל השופך דמן של רשעים כאילו הקריב קרבן
R. Zeira was certainly considered a rasha because he violated "Before a blind person thou shalt not place a stumbling block" (Levit. 19:14) in bringing Rabba to two of he most severe sins. Drunkenness regarding which it is stated that there is no greater sin (Meoros to Megilla 7b, Kol Bo and Orchos Chaim cited by Darchei Moshe and Beis Yosef respectively to Orach Chaim 695).
השכרות הוא איסור חמור ואין לך עבירה גדולה ממנו
And murder which is one of the three cardinal sins which we must die rather than violate.
We know that we have to stand to do the Avodah (see The Shulchan Aruch Orach Chaim 98:4) so Rabbah stood to kill R. Zeira.
Rabba held that the l'sheim yichud to be performed upon acts brought on by excessive consumption of alcohol on Purim should be performed standing. (See ר' משוגע).
The Gemara says year-round, he got out his bloodlust by putting people in Cherem. And whenever he would do so, he insisted he was standing on principle. (How do we know Rabbi Zeira was principal of a yeshiva? Teiku.) So he had a Pavlovian association between fulfilling bloodlust and being in a standing position, which he applied here.
Actually, Rabba stood up because Rabbi Zeira was singing Hatikvah, because he was a Zionist. How do we know that Rabbi Zeira was a Zionist? Because he told the Golem "you come from the Chaverim, [i.e. a Chaver Knesset]; go back to your earth"; if a Golem should go back to its earth out of respect for the Chavrei Knesset, Kal VaChomer the Jews should respect the Chavrei Knesset and go back to their land!
V'Ika D'amri... he stood up because he was tired of people confusing him with Rava. The same way that kindergarten teachers go around the room, poking the students in the tummy and having them say "ooh!", and on the head to say "oh!", to demonstrate he different sounds a vav makes depending on where the dot goes; so too he would stand up to show the higher-in-the-mouth vowel sound in "Rabba" (as in "Robin Hood"), then sit down to show the lower-in-the-mouth vowel sound in "Ruh-vuh."