In fact, in the original book of "The Legend of Zhen Huan", it was Huanbi who killed her. Let me talk about itIn the TV series, does Huanbi have a motive?Is there any indication that Huanbi killed Meng Jingxian?Sick seedlings Meng JingxianIn the TV series, we may be able to draw the house fight of the palace from the dialogue between Meng Jingxian and Huanbi entering the palace several times. First of all, Meng Jingxian married into the gate of the palace through the relationship of Meng Guogong, but it was only a side blessing. 
The first thing she said when she married into the door was:
"I knew it was like that. ”
Compared with the noble concubine in "The Legend of Ruyi", there is no room with the prince when he said it.
"Maybe tomorrow night, the prince will come to my house."Meng Jingxian is a little less innocent and cute, and more persistent. This attachment has buried the disaster of his day. Huanbi said for the first time after getting married: "What kind of face can she give me!.""It's just that the prince looks at her pitiful, but the prince still really loves me. We know that at the beginning, Huanbi was more favored, and as a sick seedling, Huanbi didn't take her seriously, and the prince was not very close to her. Assertion of sovereigntyThe second appearance was when Meng Jingxian accompanied King Guojun into the palace, the prince just pulled his sleeve, and she quickly grabbed the prince's hand. So that everyone in the harem of Concubine Xi and Concubine Jing can see how intimate she is with the prince and declare her sovereignty.
What is Huanbi doing at this time?Huanbi was busy with the prince's family affairs and didn't have time to enter the palace. But here is the only reason that hints that Huanbi may harm Meng Jingxian. Infatuation is like a little leaf, she also saw the scene where Meng Jingxian and the prince showed affection when they entered the palace. As soon as she came in, she said such a sentence sarcastically mocking Concubine Xi. 
alluding to Concubine Xi, she was obviously unhappy, but she couldn't reveal it. But her intention to come was not to ridicule Concubine XiShe wanted to remind Concubine Xi not to let Huanbi break her mind. 
Concubine Xi said: "Yuyin, not a stingy person. ”But Huanbi is a stingy person. Otherwise, the best sister Liuzhu would not be happy when she won the emperor's praise. She hollowed out her mind and wanted to marry the prince. After getting the prince, he slandered Zhen Huan. said that it wasn't Zhen Huan who asked her to take care of her, Zhen Huan went to the emperor so quickly, and she didn't deserve your love at all. She's so stingy, and she's so scheming. Otherwise, how could she betray Zhen Huan again and again, and use means to achieve what she wants again and again. Harm Meng Jingxian, she can do it. Suspicious and thoughtfulThe third time the situation was reversed. Huanbi came to Yongshou Palace alone, saying that she couldn't look down on Meng Jingxian pestering the prince every day. You can see that she really minds. In the past, she was also pestering the prince like this, why was Huanbi not angry in the past. It's just that recently, the prince has been better to Meng Jingxian than to Huanbi. Why?Because Meng Jingxian is quite proficient in poetry and books, and Huanbi listens to the prince talking about kites, she will only think that the prince likes kites. It can be seen that the relationship between An Lingrong and the emperor is almost the same. And when she heard that Meng Jingxian had a child of the prince, Huanbi's first reaction was.
"There is no place for her in the palace."
And if she wants to accommodate herself, the best way is to follow the example of the queen who harmed Zhen Huan's children, harm Meng Jingxian's children, or directly harm Meng Jingxian.
Huanbi did have this killing intention, but Zhen Huan persuaded her. 
Note that Zhen Huan deliberately said it here.
"King Guojun's side Fu Jin should be virtuous and virtuous. “It can be seen that Zhen Huan also knows that Huanbi has done things that harm the child and Meng Jingxian. And this is how Huanbi came back:
Note that she only said that she would not treat the child badly, but she did not respond to not harming Meng Jingxian.
Accidentally killedIt just so happened that Meng Jingxian was poisoned by Jianqiu, and she could really do what she wanted next to her. 
But this is only her motive for committing the crime, what the imperial doctor said later when he reported to the emperor.
"Xian Fujin, who was weak after giving birth, died of poison, and just passed away. ”This returns to the common sense of modern judicial law, "there is no doubt about the guilt". Where suspicion of guilt is never required at the trial stage, and it is not sufficient to prove the defendant's guilt or not to prove the defendant's innocence, he shall be presumed innocent. Many people will support the "suspicion of guilt" based on motives. The reason why it supports "there is no doubt about the guilt", a reply on Zhihu said that Banshan gave a good answer. Because from nothing, if you are wrong, you will only let go of a "bad person";If there is one, if it is wrong, not only will a "bad person" be spared, but also a "good person" will be wronged. This is the reason why I support Huanbi for not killing Meng Jingxian, so Meng Jingxian was indeed poisoned by Jianqiu and killed by mistake.