Definitely. And I don't know enough about European history, to be honest, to say definitively whether comparable atrocities were committed... but I definitely assume so, between the inquisition and crusades and Roman conquest and so on. Killing entire extended families is harsh but I know of examples in Chinese history, for example, where it happened as well.
It's all very Game of Thrones. A surviving heir complicates things. Think about the Starks getting betrayed in GoT and House Bolton coming for all the children and Stark loyalists. Otherwise you have a situation where, years later, the North rises up under the surviving Snow/Starks. Or shoot, any surviving Targ or Targ ******* - that's half the show.
Even after Sekigahara (1600), when Tokugawa Ieyasu (or "Toranaga" from the show) becomes shogun, he lets the Taiko's heir survive until 1614 when the relationship deteriorates and the heir starts gathering ronin in Osaka. Ieyasu wasn't about to let the country fall into civil war again so he comes down hard on them and anyone who supports them.
It's brutal and fascinating and tragic and has led to all kinds of great stories, plays, books, video games, etc.
A few miscellaneous tidbits from the era:
- When Oda Nobunaga (1st great unifier) is betrayed by Akechi Mitsuhide (Mariko's father), he comes for "Toranaga" as well, but he escapes when one of his vassals, Hattori Hanzo, leads him through the mountains to safety. Hattori Hanzo is usually depicted as the ultimate ninja in movies and film, but he was also a real life Tokugawa retainer.
- Many of the castles from this era, including Osaka castle, still exist in Japan today. I really hope to visit Osaka castle in particular someday.
- There's a well-known idiom in Japan, something along the lines of "what do you do if the bird won't sing?" The first great unifier, Nobunaga (known for his brutality) says "kill it." The second great unifier, Hideyoshi (the Taiko, known for his cunning) says "convince it to sing." The third great unifier, Ieyasu ("Toranaga" from the show) says "wait." I think this show did a great job showing a guy with the patience to wait and navigate 60 years of war and politics until it was finally his turn to rise to the top.
- A samurai from this era, Ukon Takayama, became a catholic saint in recent years. He was eventually exiled for his faith. The show/book make a way bigger deal out of Christianity in this period than it was (it wasn't 2/5ths of the council, for example) but it definitely played a role.