我上个星期三和一个朋友去电影院看了《子弹头列车》。

If you are just blogging online, you can do it without punctuation, but in a formal article, you need to add a “book title” mark to the title of the movie. XD

（这部电影）根据伊坂幸太郎 2010 年出版的小说《瓢虫》改编的电影。

or （这部电影）改编自伊坂幸太郎 2010 年出版的小说《瓢虫》。(改编自=adapted from...)
or （这是）根据伊坂幸太郎 2010 年出版的小说《瓢虫》改编的电影。

You can put this sentence together with the previous one.
In the news, it is common to see expressions like:
这部电影是 2022 年的美国动作喜剧，改编自伊坂幸太郎 2010 年出版的小说《瓢虫》。

正片比预告片血腥一点，但比起我很久以前看过的另一部电影更容易观看，它没有那么血腥。

I think you want to say:
This movie is gory but it is less gory than another movie I saw a long time ago.

in Mandarin Chinese, more gory is 更血腥 (更=more, 血腥=gory/bloody), but it's hard to express "less gory".
You can literally translate it as "更不血腥" （更不=less, 血腥=gory/bloody）, but it sounds unnatural.
People use "没有那么血腥" in speech. it means "not gory to that extent; not that gory".
"比起我很久以前看过的另一部电影，它没有那么血腥" means "It's not as gory as another movie I saw a long time ago".

更容易观看 means "easier to watch". It could mean that the movie's plot is easier to understand, or the movie has a clearer picture.

（是根据）伊坂幸太郎 2010 年出版的小说《瓢虫》改编的电影。

（正片/电影）比预告片血腥一点，但比我很久以前看过的另一部电影更容易观看。

更容易看懂？