As pointed out by Zodiac researchers Cragle and Druzer (among others), the recent Manson family murders of Sharon Tate, Jay Sebring, Abigail Folger, Wojciech Frykowskion and Steven Parent on August 8th 1969, along with the murders of Leno and Rosemary LaBianca on August 9th 1969, likely had a part-influence in the Zodiac Killer changing course in his style of attack. When we consider why the Zodiac Killer chose to wear a hood at Lake Berryessa, it's probably wise to examine the murders of all these nine victims when considering the choices made by the Zodiac Killer at Lake Berryessa. Therefore, it's important to review the newspaper coverage in the run up to the attack on September 27th 1969 because we know the Zodiac Killer was heavily influenced by what he read..
All the ingredients were present in these newspaper articles to shape the Zodiac Killer's Lake Berryessa attack, with the use of clothesline cord, the black hood, the bayonet-style weapon as described by Dr DE Petris at the autopsy of Cecilia Shepard, and the writing on the car door of Bryan Hartnell (to replace the front door). If the Sharon Tate & LaBianca murders part-influenced the shift from gun to knife in the Zodiac Killer slayings, why couldn't the reading of "a black hood over a head" have inspired him to create his theatrical costume at Lake Berryessa.
I am also confident that the Sharon Tate & LaBianca murders influenced the design of the December 16th 1969 Fairfield letter (with the "Bleeding Knife of Zodiac" drawing), when the Zodiac Killer wrote "This is the Zodiac speaking. I just want to tell you this state is in trouble. I will go for the government life. So don't forget me. I will kill more people than you can count. So look for more blood.". (corrected for errors). The following article (edited for conciseness) was published in the Los Angeles Times on December 16th 1969, a matter of hours before the Fairfield letter was mailed with an afternoon postmark. Therefore, a strong possibility exists that this article, featuring the Zodiac Killer, could have influenced the language and tone of the Fairfield letter. The opening sentence of the Fairfield letter stated "I just want to tell you this state is in trouble", which can be seen to be synonymous with the newspaper article that headlines with "State Furnishes List of Murders Similar to 7 Slayings Here", as well as the opening paragraph of the article that reads "State officials have provided Los Angeles police with details of 30 unsolved murders", and the sub-headline of "All Murders Logged by State". We know that the Zodiac Killer kept himself informed of murders other than his own, so wouldn't have been averse to using those newspaper articles to style his own communications (and possibly his attacks}
If the Zodiac Killer was inspired to write on the door of Bryan Hartnell's 1956 white Karmann Ghia by reading newspaper articles about the Manson murders and the writing on the front door of the Benedict Canyon home, was he also influenced by the same thing, when (in all probability) he designed the "Channel 9" letter on May 2nd 1978 by using "1,2,3,4,5,6,7 -- All Good Children Go to Heaven" on the envelope, taken from the "Helter Skelter" door at the Spahn Ranch? It originated from a Beatles song (hence the Apple logo drawn on the envelope), but by describing Susan Atkins (a member of the Manson family) in the letter as the "Judas of the Manson Family", it isn't hard to envisage that the Zodiac Killer could have been inspired by the two doors from the Benedict Canyon home and Spahn Ranch in his writings on September 27th 1969 and May 2nd 1978.